Can Group-Functional Behaviors Evolve by Cultural Group Selection?: An Empirical Test
Author(s): Joseph Soltis, Robert Boyd and Peter J. Richerson
Source: Current Anthropology, Vol. 36, No. 3 (Jun., 1995), pp. 473-494
Published by: University of Chicago Press on behalf of Wenner-Gren Foundation for
Anthropological Research
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Volume36, Number3, JuneI99S
ANTHROPOLOGY
Research.All rightsreservedOOII-3204/95/3603-0005$2.50
FoundationforAnthropological
? I995 byThe Wenner-Gren
CURRENT
ofCalifornia,
University
Davis. He has taughtat Duke Univer-
Can Group-functional
BehaviorsEvolve by
Cultural Group
Selection?
sity (i980-84) and at Emory University (i984-85) and has been a
visitingfellowat theNeurosciencesInstitute(i984) and at the
StudiesoftheUniversity
ofBielefeld
CenterforInterdisciplinary
(i99i-92). In addition to cultural evolution, his research inter-
An EmpiricalTest'
estsincludeplanktoncommunity
ecology,theecologyoftropical
lakes,and appliedstudiesofpollutedlakes.He has published,
withPeterJ.Richerson,Cultureand theEvolutionary
Process
(Chicago:University
ofChicagoPress,I985).
ofEnvironmental
Studiesat
PETER J. RICHERSON iS Professor
theUniversity
ofCalifornia,
Davis, wherehe receiveda B.S. in
in i96s and a Ph.D. in zoology(ecology)in I969. He
entomology
has beena visitingprofessor
in forestry
and environmental
studies at Duke University
(I984) as well as a visitingfellowofthe
NeurosciencesInstitute(I984) and ofthe CenterforInterdisciofBielefeld(i99i-92) and direcplinaryStudiesoftheUniversity
tor of UC Davis's Institute of Ecology (I984-90).
The presentpaperwas submitted
in finalformI7 X 94.
by JosephSoltis,RobertBoyd,
and PeterJ.Richerson
believethatsocial and culturalvariationresults
Functionalists
fromadaptationat thegrouplevel.Such explanationsare controversialfortworeasons:(i) Extensiveanalysisofmathematical
biologistssuggests
modelsofgroupselectionby evolutionary
(2) Groupextincthatgroupselectionis unlikelyto be important.
change.
evolutionary
tionsare too rareto generatesufficient
Boydand Richersonhave proposeda new modelofgroupselecmoreplausitionbasedon culturalvariationthatis theoretically
ble thangroupselectionon geneticvariation.In thispaperwe
groupformation,
presentdata on patternsofgroupextinction,
variationin New Guineawhichare consisand between-group
tentwiththeoperationofthismodel.Observedratesofgroupextinctionsuggestthata minimumof 50 to i,ooo yearswouldbe
traitunderthe
requiredforthespreadofa singlegroup-beneficial
influenceofgroupselection.This resultimpliesthatgroupselectioncannotexplainculturalchangesthattakeless than 50 to
precludea roleforgroupselecI,OOO years.It does not,however,
tionin explainingtheevolutionofhumansocietiesoverthe
longerrun.
ofAnSOLTIS is a doctoralcandidatein theDepartment
Los Angeles(Los
ofCalifornia,
oftheUniversity
thropology
Angeles,Calif.90024, U.S.A.). Bornin i962, he receivedhis B.A.
a visfromUCLA in I988 and his M.A. in i99i. He is currently
itingresearchstudentat thePrimateResearchInstituteofKyoto
femalematechoicein Japanese
wherehe is studying
University,
macaques.
at UCLA. He was
ofAnthropology
ROBERT BOYD iS Professor
of
bornin I948 and has a B.A. in physicsfromtheUniversity
San Diego (I970), and a Ph.D. in ecologyfromthe
California,
JOSEPH
i. We thankPhilipNewman,Paul Sillitoe,AndrewVayda,Mark
Allen,and Bob Rechtmanforhelp in locatingdata used in this
analysis.JoanSilk,TimothyEarle,EricSmith,Paul Allison,Lore
Ruttan,Mark Jenike,Alan Rogers,Monique Borgerhoff
Mulder,
and an anonymousreferee
providedveryusefulcommentson earlierdraftsof thispaper.Membersof the University
ofBielefeld's
CenterforInterdisciplinary
Researchprojecton the Biological
FoundationsofHuman Cultureprovideda constructively
critical
audienceforan earlyversion(specialthanksare due its director,
PeterWeingart).
Jonathan
Turnerconvincedus thatstate-level
infromtribalones.
stitutionsare different
Many anthropologistsexplain human behaviorand social institutionsin termsof group-levelfunctions(Rap-
paport I984, Lenski and Lenski I982, Harris I979, Radcliffe-BrownI952, Aberle et al. I950, Malinowski I984
[I922], Spencer i89i). According to this view, beliefs,
behaviors,and institutionsexist because theypromote
the healthyfunctioningof social groups.Such functionalists believe thatthe existenceof an observedbehavior
or institutionis explained if it can be shown how the
behavioror institutioncontributesto the health or welfareof the social group.Most functionalistsin anthropologyhave not explainedhow group-beneficial
beliefs
and institutionsarise or by what processes they are
maintained (Turner and Maryanski I979). When func-
tionalistsdo providea mechanismforthe generationor
maintenanceof group-leveladaptations,it is usually in
termsof selection among social groups.2Functionalists
2. Some authors(e.g.,HarrisI979) have suggestedthatthe selfinterested
choicesofindividualswill resultin group-beneficial
behavior.However,thisclaimis notcogent-group-beneficial
behaviorwill notresultfromindividualchoiceexceptas a side effect
of
otherprocessesor in certainlimitedcircumstances.
Forexample,
manyauthorshave suggestedthatfoodtaboosexistbecause they
of ecologicalresources.To keep things
preventoverexploitation
simple,let us supposethatindividualsmustchoose to observea
tabooornotandthatindividualswhoobservethistaboo
particular
andnutritious
fooditem.Choosingto ignorethe
forgoa satisfying
taboo has a positiveeffecton individuals'own welfareand, by
assumption,
a negativeeffect
on thewelfareofthegroup.However,
unless the groupis verysmall,the personaleffectwill be much
on thegroup,andthuschoosingto ignorethe
largerthantheeffect
taboowillbetterserveindividuals'goals,eveniftheirgoalsinclude
thewelfareofthegroup.This effect
is at theheartofbothrationalstrategy
andevolutionary
arguments
againsttheeasydevelopment
ofgroup-beneficial
behavior.The effect
is nota matterofcognitive
capacity,as writerssuchas Harrisseem to imply.Rationalstrategistsare assumedto have unlimitedcognitivecapacity,whereas
evolvedcreaturesare the productsof blindselectivesorting,but
the essentialproblemis the same; both rationalstrategists
and
evolvedcreaturesare expectedto act in theirown self-interest.
behaviormay resultfromself-interested
Group-beneficial
individualchoiceundercertaincircumstances.
First,sinceindividual
andgroupbenefitareoftencorrelated,
individualchoicemayoften
producegroup-beneficial
outcomesas a sideeffect
(seeSugdenI986
forseveralexamples).Second,marketswill lead to an "efficient"
allocationofeconomicresourcesifthestateorsomeotherexternal
473
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474
CURRENT
ANTHROPOLOGY
Volume 36, Number 3, fune 1995
believe that societies have many functionalprerequivalues,
sites. Social groupswhose culturallytransmitted
beliefs,and institutionsdo not provideforthese prerequisitesbecome extinct,leavingonlythosesocietieswith
functionalcultural attributesas survivors.We referto
this processas "culturalgroupselection" because it inof culsurvivaland proliferation
volves the differential
turallyvariablegroups.
Culturalgroupselectionis analogous to geneticgroup
selectionbut acts on culturalratherthangeneticdifferences betweengroups.This distinctionis important.We
will arguethatculturalvariationis moreproneto group
selection than genetic variationand that this may explain whyhuman societies,in contrastto those ofother
animals, are frequentlycooperativeon scales farlarger
thankin groups.More generally,recenttheoreticalwork
on the processes of culturalevolutionshows that there
are many parallels between culturaland geneticevolution but also some fundamentaldifferences(Durham
tence of group-beneficial
traits: maintenance of variation among groupsand rate of adaptation.
Group-functionalexplanations may be in conflict
withthe factthathuman choices are at least partlyselfinterested.To the extentthat they can evaluate alternative beliefs and attitudes,self-interested
organisms
should adopt only beneficialattitudesand beliefs and
rejectthose that are individuallyharmful.Thus, beliefs
thatare costlyto the individualshould diminish,while
beliefsthat are beneficialto individualsshould spread.
Extensivetheoreticalanalysissuggeststhatgroupselection can counteractthis process only if groupsare very
small and migrationamonggroupsis verylimited(Eshel
I972, Levin and Kilmer I973, Wade I978, Slatkin and
Wade I978, Boormanand LevittI980, Wilson I983,
Aoki I982, Rogersi990). As a result,most evolutionary
biologistsand social scientistsinfluencedby them (e.g.,
Chagnon and Irons I979) reject functionalistexplanations.
andFeldI985, Cavalli-Sforza
Furthermore,
Hallpike (I986) has argued that group
I99I, BoydandRicherson
man I98I, Pulliam and DunfordI980). To date,empiri- extinctiondoes not occur oftenenough to justifyfunccal investigationsfocused on these processes are few tionalistexplanations.Group selection worksby elimi(but see, e.g., Cavalli-Sforzaet al. I982). In additionto natingthose societies thathave deleteriouspracticesor
conductingempiricalstudiesspecificallydesignedto in- institutions.If it takes a particularnumber of extincvestigatethese processes,it is possible to use many of tions to eliminatea deleteriousritualform,then it will
the data collectedbysocial scientistsforotherpurposes. take a greaternumberto eliminatethe deleteriousritual
Here we use a small part of the veryrich ethnographic formand a deleteriousmarriagepractice. Still further
to testthe empirical extinctionswill be requiredto cause otheraspectsofthe
recordproducedby anthropologists
societyto become adaptive.Hallpike arguesthathuman
plausibilityof the process of culturalgroupselection.
byCampbell(i965, I975, I983), cul- societies do not have high enough extinctionrates for
As is emphasized
attributesto be
turalgroupselection requiresthat (i) therebe cultural groupselectionto cause many different
differencesamong groups, (2) these differencesaffect adaptiveat the grouplevel simultaneously.
In the face of these objections,is thereany justificaof groups,and (3) these difpersistenceor proliferation
hypothesesseriously?
ferencesbe transmittedthroughtime.Ifthesethreecon- tion fortaking group-functional
ditionshold, then,otherthingsbeingequal, culturalat- Here we describea theoreticalmodel and presentsuptributeswhich enhance the persistenceor proliferation portingdata which show that a role forculturalgroup
of social groupswill tend to spread.There is no guaran- selection should not be ruled out. Boyd and Richerson
pow- (I985:chaps. 7 and 8; iggoa, b) have analyzed mathetee, however,thatthis processwill be sufficiently
to matical models of groupselection acting on culturally
act
which
social
other
processes
erfulto overcome
produceotheroutcomes. There are two problemswith transmittedvariation and have shown that cultural
culturalgroupselection as an explanationforthe exis- groupselectionwill workifcertainkey assumptionsare
met. Ethnographicdata fromPapua New Guinea and
IrianJayagive credenceto some ofthe assumptionsthat
suchas air pollution underpinthe
externaleffects
enforcescontracts,
authority
group-selectionmodel. These data also
The
anda numberofotherconditionsaresatisfied.
arenotpresent,
onlyin the sense thatno one can be made allow us to estimatean upperbound on the rateofadapallocationis efficient
betteroffwithoutsomeoneelse'sbeingmadeworseoff-thedistri- tationthat could resultfromgroupselection.We argue
to the that these data suggestthat groupselection is too slow
deleterious
butionofwealththatresultscouldbe extremely
survivalof the society.Clearly,most aspectsof cultureare not to be used to justifythe commonpracticeofinterpreting
societies.
regulatedby marketsor prices,even in contemporary
the detailedaspects ofparticularculmay also lead as group-beneficial
Third,rationalplanningby leadersor institutions
outcomes.Whiletheextentto whichpolitical tures.However,the data do not exclude the possibility
to group-beneficial
can everbe modeledas actingin thecommoninterest thatgroupselectionmay account forthe gradualevoluinstitutions
is debatable,it is clear thatmost aspectsof cultureare not the tion of some group-leveladaptations,such as complex
resultofrationalplanning.Finally,individualsmaychoosegroup- social institutions,over many millennia.
if theyvalue those activitiesfortheirown
beneficialactivities
sake, not because theybenefitthe group(Margolisi982, Batson
For example,men may fightto defendthe groupif they
I99I).
how How Group Selection Can Work
valueheroismin battle.However,one is leftwithexplaining
the explanation
men come to have such preferences-otherwise,
behaviorsbecausetheylike
is thatpeoplechoosegroup-beneficial
to do so. Thus,we do not denythatpeoplemakegroup-beneficialWe beginwiththe premisethatindividualsacquire varichoices.We areclaimingthatwhensuchchoicesoccurtheycannot ous skills,beliefs,attitudes,and values fromotherindichoice.
viduals by social learningand that these "cultural varibe theresultofmainlyself-interested
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SOLTIS,
BOYD,
AND
RICHERSON
Group Selection: An Empirical Test | 475
poorernutritionalstaants," togetherwith theirgenotypesand environments, resourcesand suffersignificantly
determinetheirbehavior.To understandwhypeople be- tus as a result. Furthersuppose that social groups are
have as they do in a particularenvironmentwe must occasionally disruptedand theirmembersdispersedto
know the skills, beliefs,attitudes,and values that they otherlocal groupsand thatthe rateat which this occurs
have acquired fromothersby cultural inheritance.To dependson the overallwelfareofthe group.Such disrupdo this we must account forthe processes that affect tion and dispersalmay be the result of population deculturalvariationas individualsacquire culturaltraits, cline, social discord,or the actions of aggressiveneighuse the acquired informationto guide behavior,and act bors. Poor nutritionalstatus will contributeto these
as models for others. What processes increase or de- risks. Thus, according to our hypotheticalexample,
crease the proportionof personsin a society who hold groupswith less restrictivefoodtaboos will, on average,
particularideas about how to behave?Here we will con- be more likely to be brokenup and dispersed.Finally,
sider two kinds of processes: biased cultural transmis- supposethatas some groupsdeclineand disappear,other
groupsgrowand eventuallydivide,formingnew groups,
sion and selection among social groups.
Biased culturaltransmissionoccurs when individuals and thattherateat which this occursincreaseswiththe
adopt some variantsrelativeto others.In- overallwelfareofthe group.Thus, the growing,dividing
preferentially
dividualsmay be exposedto a varietyofbeliefsorbehav- groupswill tend to have more restrictivefood taboos
foodtabooswill tend
iors,evaluate these alternativesaccordingto theirown thandecliningones,and restrictive
goals, and preferentiallyimitate those variants that to spreadas a resultof selection among groups.Others
seem best to satisfytheirgoals. Ifmanyofthe individu- have proposed at least implicitlysimilar models (e.g.,
als in a populationhave similargoals, this processwill Peoples I982, Divale and Harris I976, Irons I975).
This model of groupselection differs
fromthose anacause the culturalvariantsthat best satisfythese goals
to spread.For example,ifthe two variantsare more and lyzed in populationbiologyin thatbiased transmission
less restrictiveformsoffoodtaboos and individualspre- maintainsvariationamong groups.Biologistshave been
ferthe broaderdiet thatresultsfromthe less restrictive concernedwithwhethergroupselectioncould allow the
variant, then that variant will spread. This process, evolutionofaltruisticbehavior.In thesemodels,natural
which is importantin the spreadofinnovations(Rogers selectionacts againstaltruisticbehaviorin everygroup,
I983), oftentendsto cause groupslivingin similarenvi- and this selection process tends to reduce variation
among groups. The only process creating variation
ronmentsto have similarbehaviors.
However,biased culturaltransmissioncan also main- among groupsis geneticdrift,a veryweak force.Thus,
betweengroupsofpeople livingin simi- groupselectioncan have littleeffectbecause groupsare
tain differences
lar environments.This can occur in two ways: First,a geneticallyverysimilar.In the model outlinedhere,it
beliefor behaviormay be more attractiveif it is more is assumed that various formsof biased transmission,
widelyused thanthealternatives.Many social behaviors potentiallyvery strongindividual-levelforces,act to
have this character.For example,iffoodtaboos are used maintain differences
among groupsand groupselection
as ethnic markers,then in a groupin which the more can predominate.
The formof group selection just outlined can be a
restrictivetaboo predominatesindividualsmay choose
thattaboo overtheless restrictiveone because thesocial potentforceeven if groups are usually very large. For
benefitscompensateforthenutritionalcosts. Game the- a favorableculturalvariant to spread,it must become
orysuggeststhat many kinds of social interactions,in- common in an initial subpopulation.The rate at which
cludingbargaining,contests,and punishment-enforcedthis will occur throughrandomdriftlikeprocesses (Canorms,will generatean astronomicalnumberofalterna- valli-Sforzaand Feldman i98i) will be slow forsizable
tive equilibria. Second, when individualsare unable to groups (Lande i986). However, this need occur only
evaluate the meritsof alternativevariants,theymay in- once. Thus, even if groupsare usually large,occasional
stead use a simple rule of thumb such as adoptingthe populationbottlenecksmay allow groupselectionto get
most common variant.This conformistformof biased started.Similarly,environmentalvariationin even a few
transmissioncauses the more common variant to in- subpopulations may provide the initial impetus for
crease. For example,if the majorityof a groupobserves groupselection.Some environmentsmay lead groupsto
traitsbecause theyare also indiadopt group-beneficial
the more restrictivetaboo, it will tend to increase.
or conformity viduallyadvantageous.These practicesmay thenspread
When either common-type-advantage
maintainsdifferences
amonggroups,groupselectioncan by groupselection into environmentswhere theyhave
be an importantforce.Considera largepopulationsub- only a group advantage. For example, restrictivefood
dividedintomanysmaller,partiallyisolatedgroups.Sup- taboos may arise in a veryheterogeneousenvironment
pose that biased cultural transmissionmaintains cul- in which it is importantforindividualsto specialize in
tural differencesamong these groups despite frequent narrow-range
food-procurement
strategiesand onlylater
contactand occasional intermarriage
and thatthese cul- spreadby groupselectionto less heterogeneousenvironturaldifferences
affectthe welfareof the group.For ex- mentswhere theymainlyfunctionto protectresources
ample,groupsin which restrictivefoodtaboos are com- againstthe tragedyof the commons.
mon may tend to harvest game at approximatelythe
Unlike manygeneticmodels,thisformofgroupselecmaximumsustainableyield,while groupsin which less tion does not require that the people who make up
restrictivetaboos are common overexploittheir game groupsdie duringgroupextinction.All that is required
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476 1 CURRENT
ANTHROPOLOGY
Volume 36, Number 3, June1995
is the disruptionof the group as a social unit and the
dispersal of its members throughoutthe metapopulation.Such dispersalhas the effectofculturalextinction,
because dispersingindividualshave little effecton the
frequencyof alternativebehaviorsin the future;in any
one host subpopulationthey will be too few to tip it
fromone equilibriummaintainedby conventionor conformityto another.
Cultural groupselection is verysensitiveto the way
in which new groups are formed.If new groups are
mainlyformedby individualsfroma single preexisting
group,then the behaviorwith the lower rate of extinction or higherlevel of contributionto the pool of colonists can spreadeven when it is rarein the metapopulation. If,instead,new groupsresultfromthe association
of individualsfrommany othergroups,groupselection
cannot act to increase the frequencyof rarestrategies.
EmpiricalEvidence
To justifyusing this model of culturalgroupselection
we need data that allow us to answer threequestions:
i. Do groupssuffer
disruptionand dispersalat a rate
highenoughto account forthe evolutionof any important attributesof human societies?
2. Are new groupsformedmainlyby fissionin groups
thatavoid extinction?
3. Are theretransmissibleculturaldifferences
among
groups that affecttheir growth and survival, and do
these differences
persistlong enoughforgroupselection
to operate?
To addressthese questions we presentdata on group
extinctionrates,groupformation,and culturalvariability drawnfromthe ethnographicliteratureof Irian Jaya
and Papua New Guinea. We have chosen this area because it offershigh-qualityethnographicdescriptionsof
peoples thathad not been pacifiedbya colonial administration.Colonialism is suspected by some to increase
ratesofintergroupconflictin statelesssocieties,casting
doubt on data from areas like the American Plains,
New Guinea
wherecontactpredatedgood ethnography.
is unique in the amount of good ethnographyobtained
withina fewyearsof firstcontactwith complex societies. We have focusedon pre-statesocieties because they
are characteristicof more of human historythan more
complex societies and the basic institutionsof human
societies evolved understateless conditions.
We have made an effortto sample as many ethnographiesas possible,focusingon those dealingwithprecontact warfareamong indigenous peoples. We have
chosen to focuson warfareonlybecause it is a conspicuous way in which groupsbecome extinctand is likely
to be recorded.Even where defeatin war is the proximate cause of an extinction,a varietyof otherfactors
may have precipitatedthe eventby causingthe defeated
group to decline in numbers.Extinctionthroughwar
may be the commonfateofgroupswhich have declined
forsome otherreason.
We definea groupas a territorial
populationthat can
conductwarfareas a unit. An extinctionis said to occur
when (i) all membersof a groupare killed or (2) membersof a groupare assimilatedinto anothergroupeither
whollyor in part.When a groupis routedfromits territorybut remains intact as a social unit (or its fate is
unknown),then a forcedmigration,not an extinction,
is said to have occurred.
GROUP
EXTINCTION
To estimate the rate of group extinctionfora region,
threetypes of informationare needed: (i) the number
of extinctions,(2) the numberof years over which the
extinctionstook place, and (3) the number of groups
amongwhich the extinctionstook place. We were able
to assemblethisinformation
forfiveregionsin IrianJaya
and Papua New Guinea.
The Mae Enga. The Mae Enga live in the Central
WesternHighlands,where population densityaverages
but reaches densitiesof over ioo
40 to 43 persons/km2
persons/km2(Meggitti962:1 58; I977:i).
The immediate causes of war (Meggitt I977:I3) are land disputes
(58%), otherpropertydisputes (24%), homicide (I5%),
and problemsrelated to sexual jealousy (3%). Meggitt
recordeda 5o-yearwarfarehistoryforI4 Mae Enga clans.
In the 29 conflictsforwhich the outcome was known,
therewere 5 extinctions.Extinctionsdid not resultfrom
the killingof all groupmembers;routedclan members
were forcedto disperse and find refugeamong other
clans, often with kin (I977:I5, 25-27).
There is evidence
that these immigrantsbecame culturallyassimilated
into theirhost group,usually withina generation(Meggitt I965:3I-35).
Rapid assimilation occurredbecause
true clan membersreceivedunqualifiedland rights,as
well as economic, ritual,and militaryaid. As Meggitt
(I977:i9o)
notes, "Members of defeated and dispersed
groupswho have gone to live elsewherehave goodpolitical and economic reasonsnot to draw attentionto their
immigrantstatus but instead tryforrelativelyrapidabsorptioninto the host clan.... In consequence,the identitiesof extinguishedclans or subclans are soon lost to
public knowledgeand in time such groupsdrop out of
the genealogiesof theirformerphratries."
The Maring. The Maring live in the Central Highlands,an area of relativelylow populationdensities,averaging less than 2o persons/km2 (Vayda I97I:22).
Wars
are usually triggeredby a murderor attemptedmurder
(56% ofcases). The remaining44% are foughtoverland,
women, or theft(I97I:4). Vayda's warfarehistoryconcerns 32 clan-clustersand autonomous clans and has a
depthof about 5o years (AndrewVayda,personal communication). He mentions I4 wars in which victims
were routedfromtheirterritories.
Only in one case was
therea clear extinction;the othergroupseventuallyreturned.However,in two of these cases routedclans reclaimed theirterritory
onlywith the help of the Australian police and probablywould have become extinct
otherwise.Rappaport(i967:26) explains that members
of vanquishedgroupswho findrefugein anothergroup
do not maintaintheirautonomy:"the de factomember-
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SOLTIS,
BOYD,
AND
RICHERSON
Group Selection: An Empirical Test | 477
ship of the livingin groupswith which theyhave taken indicates that his warfaredata are most complete for
refugeis convertedeventuallyinto de jure membership. only 8 districtsin the area but mentionssome 24 disSooner or later the groupswith which theyhave taken trictsin his accounts of warfare.
The Tor. The Tor regionis located on the northern
up residencewill have occasion to plant rumbin,thus
rituallyvalidatingtheirconnectionto the new territory coast of Irian Jaya(Oosterwal i96i). No densityfigures
are provided.Oosterwal recordeda 40-yearhistoryfor
and theirnew group."
The Mendi. The Mendi live in the SouthernHigh- the 26 tribal territoriesin the Tor region.Four tribes
lands, where population density is i8 persons/km2 sufferedextinctioneitherthroughpeaceful absorption,
(Meggitti965:272). Ryan (i959) describes,fora 5o-year militarydefeatand dispersal,or outrightextermination
In one oftheextinctions,
Oosperiod,the historyof clan degeneration,extinction,and (Oosterwali96i:2i-26).
new groupformationfora groupofnine clans known as terwalis clear about the culturalassimilationof the exthe Mobera-Kunjop.In this periodtherewere threeclan tinct group: "Formerlythe Mander language was only
extinctions.In two cases, the clans were routedby war- spoken by the Mander, but since the Foja have lived
fareand absorbedbyothergroups;in the thirda degener- together with the Mander, they have adopted the
Mander language entirely.Save fora small numberof
atingclan was eventuallyabsorbedby anotherclan.
In two cases, vanquishedgroupsdid not sufferdisrup- words,theseFoja do not recollectany moreoftheirown
tion but managed to remain functioningas an intact language. Their kinship terminologyis also identical
subclan in their host group. Ryan (I959:27I) suggests with that of the Mander" (p. 23).
Table i summarizes extinctionrates forthe five rethat such accretionarysubclans eventuallybecome assimilatedinto theirhost clan: "The refugeegroup,con- gionsforwhichtherewere enoughdata to computesuch
sistingof sub-clan brothersand theirfamilies,may be estimates. We assume that the number of groups relarge enough to assume the immediatestatus of a sub- mains constant,which means that each extinctionis
clan.... Once the people have been accepted,granted followedby an immediaterecolonization.To the extent
land, and have settleddown, thereis almost no further that this assumptionis wrong,extinctionrates will be
made betweenthemand the originalsub- higher.We foundno ethnographieswhich yieldedan exdifferentiation
clans." However,individualnonagnatessufferdiscrimi- tinctionrate of zero. In our sample, the percentageof
nation frommembersof theirhost clans (Ryan i959). groupssuffering
extinctioneach generationrangesfrom
They are less likely to receive bridewealth support i.6% to 3I.3%.
It seems likely that otherareas in New Guinea had
(which normallycomes fromfellow subclan members)
than are true group members,and thereforerefugees similar group extinction rates. There is mention of
have reason to want to assimilate into theirhost group: group extinctionin 54% (i5/28) of the societies sam"Althoughit is assertedthatacceptanceis complete...
pled. This is no doubt an underestimate,because the
marriagefiguresindicate that non-agnaticmen tend to failureto mentionan extinctionin an ethnographicacmarrylater than agnatic clan members,more of them count of warfaredoes not necessarilymean that extincmarryonly once, and more of them have only one wife tions never occurred.In 89% (25/28) of the societies
at a time" (p. 269).
sampled,thereis mentionof eithergroupextinctionor
The Foreand Usufura.Berndt(i962) recordeddetailed forcedmigration(see table 2). The near-ubiquityof exdescriptionsofwar involvinggroupsin fouradjacentlin- tinctionand forcedmigrationin the ethnographic
record
guisticregionsof the EasternHighlands-the Fore,the suggests that high rates of extinctionwere common
Usufura,the Jate,and the Kamano. Forepopulationden- throughout
Papua New Guinea and IrianJayabeforepacsity is approximatelyi5 persons/km2and that of the ification.
(Berndti962:20). No values are
Usufura27 persons/km2
given for the other linguisticgroups. Berndtrecorded NEW GROUP FORMATION
one extinctionduringthe ten-yearperiodprecedinghis
research.The group was routed in warefareand dis- Group selection is most effectivewhen new groupsare
districtsin the area. The made up of membersof a single existinggroup rather
persed into several different
numberofgroupsinvolvedis slightlyambiguous;Berndt thanofmembersofmanydifferent
groups.Ifnew groups
TABLE
I
Summaryof Group ExtinctionRates forFive Regions of Papua New Guinea and Irian faya
Region
Mae Enga
Maring
Mendi
Fore/Usufura
Tor
% Groups Extinct
Every 25 Years
Groups
Extinctions
Years
I4
32
9
I-3
5
50
50
I7.9
I.6-4.7
26
4
IO
3I.3-IO.4
8-24
3
I
50
40
I6.7
9.6
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Source
Meggitt (I977)
Vayda (I97I)
Ryan (I959)
Berndt(i962)
Oosterwal (i96i)
478 1 CURRENT
TABLE
ANTHROPOLOGY
Volume 36, Number 3, JuneI995
2
Mentionsof Group Extinctionand ForcedMigrationin Papua New Guinea and Irian Jaya
People
Mae Enga
Huli
Melpa
Raiapu Enga
Wola
Maring
Ok
Extinction
+
+
+
+
+
+
Kuma
Chimbu
+
Jate
Fore
+
Usufura
Auyana
+
Arapesh
+
Kukukuku
Gahuku
Abelam
Mailu
Kiwai
Dugum Dani
Ilaga Dani
+
+
Bokondini-Dani
+
+
+
+
+
Berndt (i962:242)
+
+
+
+
+
Brown and Brookfield (I959:4I,
Berndt (i962:.253,
Berndt (i962:236,
Robbins (i982:2I3-I4)
+
Lea (i965:i96,
+
+
Tuzin (I976:63)
Saville (i926)
205)
Landtman (I970[I927J:I48-49,
+
Heider (I970:II9-22)
+
Sillitoe (I977:76)
+
+
Sillitoe
Morren i986:269-70), but
Vayda I97I:I7;
Meggitt(i962, i965) and Ryan (i959) providethe most
detailed descriptionsof new group formationin two
highlandsocieties.
The Enga have a nested hierarchyof patrilinealdescent groups. The phratryis the most inclusive, followed by the clan, the subclan,the patrilineage,and the
in thehierarchymaygrowor
family.Groupseverywhere
decline over time,generatedaughtergroups,or become
absorbedbyothergroups:"Groupsmay emerge,increase
functions,and in doing
in size and take over different
so achieve higherstatus by becomingco-ordinatewith
groups that previouslyincluded them. In absorption,
6i, 263-65)
260-6I)
Blackwood (I 978: io2)
Read (I955:253-54)
+
278-79)
25I, 257)
+
+
are formedwhen a single group generatesa daughter
groupfromamong its own members,then the daughter
groupwill preservethe culturalvariantscommonin the
mothergroup.Culturalvariantswhich facilitatedaughter-groupformationwill become more common in the
regionas a whole.
Societies in Irian Jaya and Papua New Guinea are
characterizedby a segmentarysocial system(Langness
i964). When membersof a social groupbecome too numerous, the group may split into two similar groups.
Conversely,when members of a social group become
too few, they may be absorbed by anothergroup at a
lower segmentarylevel (Brown I978:I84-85, i87-88).
There are numerous anecdotal accounts of new group
I977:79;
Meggitt (I 977: I4)
Glasse (i959)
Strathern (I97I:55-56,
67)
Waddel (I972:37, i86, 263-65)
Sillitoe (I977:79)
Vayda (I97I:II-I3)
Morren (i986:266-67, 272-73,
Reay (I959:7, 27, 32)
+
+
I959:57;
formation
(e.g.,Brownand Brookfield
Source
+
+
+
Jale
Kapauku
Tor
Jaqai
Marind-Anim
Bena Bena
Migration
204)
Sillitoe (I977:77)
Koch (I974:79)
Pospisil (i966)
(i96i:2i-26,
Boelaars (I98I)
Ernst (I979:36)
Langness (i964:I74)
Oosterwal
48)
groupsthatare decreasingin numbershave to relinquish
particularfunctionsand descend to a lower level in the
hierarchy.... If the decline continues,the groupseventuallyvanish" (MeggittI965:79). For a groupto achieve
or retaina particularposition in the hierarchy,it must
contain enough membersto performthe functionsappropriateto that position. For example, fromI900 onward,the populationof one Enga clan began increasing
noticeablyuntil one ofits two subclans could no longer
supportitselfon its shareofland and beganencroaching
on a neighboringclan's territory
(Meggitti965:62-63).
In skirmisheswith the neighboringclan, the subclan
functionedas if it were a sovereignclan, fightingand
negotiatinghomicide payments independentlyof the
second subclan, which was itself tryingto expand in
anotherdirection.Eventuallymembersof the two subclans settled at opposite ends of the clan territory
and
behavedas membersof separateclans by intermarrying.
Meggitt(i965:78-79) gives an account of two Laiapu
Enga phratriesdemonstrating
extinctionand new group
formation.Each phratrywas initiallymade up of four
territorialclans. One expandingclan of phratryA attackedand killedmanymembersoftwo clans ofphratry
B. The survivorsofthe two clans fledto otherclans, and
the victorious clan occupied the abandoned territory.
This successfulclan was becomingso largeas to achieve
subphratrystatus (MeggittI965:79). Ryan (I959) gives
similaraccounts ofgroupextinctionand new groupfor-
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SOLTIS,
BOYD,
AND
mation in the Mendi Valley. When clans become too
populous, they expand into new territoryand an offshoot subclan occupies it. The breakawaysubclan attains clan statusas it takes on more and morefunctions
appropriateto a clan.
CULTURAL
VARIATION
AMONG
GROUPS
RICHERSON
Group Selection: An Empirical Test | 479
paint which is applied to the bodies of novices, except
fortheir"female" parts.In communitiesof the Telefolmin tribe,however,the red paint signifiesfemalemenstrual blood. In fact, menstrual blood is sometimes
added to the concoction, a practice which would be
"completelydestructive"to the integrityof the Faiwolmin rituals.
Modes in which cosmological ideas are communicatedalso differ
amongOk communities.The Baktaman
know almost no mythsat all. A peripheralOk community,the Mianmin, has a largercorpus of myths,but
these are not centralto theirritual events. The BiminKuskusmin,in contrast,have an abundance of myths
which are integratedinto ritual (BarthI987:5-6).
Theories of conception differamong communities
Members of the Baktaman and
(Barth I987:I3-I5).
neighboringcommunitiesbelieve that childrenspring
frommale semenwhichis nourishedin themother.Telefolminmales believe that childrenare createdfroma
fusionof male and female substances; females believe
thata fusionofmale and femalesubstancescreatesonly
the fleshand blood of a child,while the female's menstrualblood alone formsthe bones. Othercommunities
are characterizedby still different
theoriesof conception.
The Faiwolmin. Variation among communities
withinthe Faiwolmin tribalarea of the Ok regionmay
providean example of culturalvariationthat is linked
Groupextinctionand groupfissionwill lead to cultural
change only if there are transmissiblecultural differences that affectthe extinctionrate or the proliferation
there is little evidence about the
rate. Unfortunately,
amount of cultural variation among local groups because so few ethnographersstudymore than one local
there is even less evidence about
group.Furthermore,
betweenlocal groupsare relatedto indihow differences
vidual and group fitnessin New Guinea ethnography,
althoughthereis quite good evidence fromotherareas
that such variationexists (e.g., Kelly's [I985] study of
the causes of Nuer expansion at the expense of the
Dinka). Nor is thereevidence about how long such differencescan persistin New Guinea groups.Archaeological and linguisticdata fromsmall-scale societies elsewheredocumentmany examples of groupexpansionby
cultures with more effectivesocial organization in
which the differencespersistedfor many generations
duringthe expansionaryphase (e.g.,Bettingerand Baumhoff's [i982] studyof the Numic expansionfromsouthto groupsurvival.Barth(I97I; cf.MorrenI984) argues
easternCaliforniaacross the GreatBasin).
Here we reviewthreedetailedstudiesofculturalvari- that more elaborate,communal rituals and specialized
ation amonglocal groupsin New Guinea. Two of these cult houses lead to morecentralizedcommunityorganistudiesfocuson the Montain Ok of Papua New Guinea, zation which increasesthe survivabilityof the commuwhile the thirdcovers the lowland Tor regionof north- nities embracingthem and that communitieswith less
ern Irian Jaya.Each of these studies suggeststhat there elaborateculturalformsand more dispersedsettlement
patternsare more likely to become extinct.Withinthe
is substantialculturalvariationamong local groups.
The Mountain Ok. The MountainOk occupythe cen- Faiwolmintribalarea,ritualorganizationand specializater of New Guinea and are made up of nine "tribes" tion findtheirmost elaborateexpressionin the centralized communities
Male initiation
(BarthI97I:I79-8I).
based on ethnolinguisticaffinities(Morren I984:i8o8i). Within these tribes are endogamous "communi- is organizedin seven gradesthroughwhich males pass
ties," sometimescomposed of severalexogamousclans. as age-sets.In westerncommunitiesthereare foursuch
Only i 5% of marriagestake place betweenmembersof grades,and in the southeasterncommunitiestheyrange
fromfourto one (p. I85). Differentritualstake place in
communities(BarthI97I:I76).
different
Ritualpracticeand beliefvaryconsiderablyfromcom- specialized cult houses. Most Faiwolmin communities
munityto community.Ritual knowledge,surrounded contain threepermanentcult houses as well as a comby secrecy,is fullysharedby only a few eldersin each munal men's house. As one moves east and southward
community.It is transmittedat male initiations,where fromcentralFaiwolmin,the numberof cult houses deit is rationedout to initiatesin steps. Bartharguesthat clines. Most of the southeasterncommunitiescontain
communitiesdiverges onlyone permanentcult house, and some performinitithe ritualknowledgeof different
because of errorand innovationon the part of the few ations in temporarystructures.
There is also variationin social organizationamong
personswho controlit. This producesintergroupvariaof important Faiwolmin communities,followinga similar west-totion in such thingsas the interpretation
ritualsymbols,theuse ofmythsin ritualcontexts,theo- east patternof decreasing centralization(Barth I97I:
ries of conception,and the emphasis on symboliccon- I84-8 6). The centralizedcommunitiesofthe Faiwolmin
formcompactvillages around several typesof semiperstructionsof human sexualityin ritual(BarthI987).
Sacred objects used in the initiationritual take on manentcult houses, and severalexogamousclans make
differentsymbolic meaning in differentcommunities up an isolated,largelyendogamouspoliticalunit. In the
(Barth i987:4-5). For example, fat from a wild male east the populationis dispersedwithinthe community
boar is emphatically"male" among both the Bimin- territory,
shiftinghousehold locations at intervalsbeKuskusminand Baktaman of the Faiwolmin tribe.The cause of soil depletionor fearof sorcery.
Accordingto Barth,"The dispersedpatternwithout
pig's fatis mixed with various substancesto forma red
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480
I CURRENT
ANTHROPOLOGY
Volume 36, Number 3, JuneI995
the cult houses . . . clearlyorganizesa smaller populationfordefense;and theirhistoryofdisplacementwould
seem to demonstratethis disadvantage"(p. I89); "the
greatercentralizationclearlyalso offersmilitaryadvanexpantagesand has resultedin conquest and territorial
sion of the more highlycentralizedgroupsin a general
south-eastwarddirection" (p. i86). He argues that the
elaborate rituals and the concomitantcommunal centralizationwere firstintroducedto the Faiwolmincommunitiesfromthe northwest,and the diffusionofthese
culturalformscreated cultural variationamong them.
Finally,selectionamonggroupsincreasedthe frequency
the highestfitnesson
ofthose culturalformsconferring
groups(p. i88):
nores the generationalcriterionto a fargreaterextent.
In contrastto those of the previoustwo areas, cultures
in the thirdregionhave a stronggenerationalaspect in
theirterminology.There is also variationwithin each
of these threebroad areas. For example, the cousin terminologyoftheBerrickis oftheHawaiian type(all cross
and parallel cousins called by the same termsas those
forsisters),while the Waf and Goeammer (of the same
culturearea) use the Iroquois type(FaSiDa and MoBrDa
called bythe same termsbut terminologically
differentiatedfromparallelcousins and fromsisters,parallelcousins commonlybut not always classifiedwith sisters).
Althoughit is difficultto show that the particular
groupextinctionsthat we have counted forthe fiveregions are due to persistentculturaldifferences,
thereis
The distributionof [cultural]formsis thus generated
abundantevidence in New Guinea and elsewhere that
by a numberof simultaneouslypartlyindependent
cultural differencesdo lead to the success of some
processes.A process of diffusionfroman innovation
groupsand the decline of others.For example, among
centre. . . seems to be takingplace. Simultaneously,
the Fore the practice of mortuarycannibalism caused
the organizationof local culturaltransmissionis
the spreadofthe deadlydisease kuru.Accordingto Dursuch that both loss and improvisationoccur and
ham's
(I99I:4II-I3)
accountofthisepisode,ritualcannew local variantsemerge.Differentritualformsimnibalismwas originallyadoptedby Fore women as a recommunitytypes;these again confront
ply different
sponse to a shortageof game. Nevertheless,the spread
each otherin warfareand compete and replace each
of the disease as a by-productof this ritual innovation
otheron the basis of theirunequal defensiveand ofthreatenedFore groups with extinctionuntil modern
fensivecapacities.
medical teams intervened.This case points up the amIf Barthis correct,this is an example of groupselec- biguous role of rational choice in the group-selection
tion increasing the cultural variants which enhance process. Individual calculation of advantagemay often
groupsurvival.He considersthe alternativehypothesis run counter to group advantage,especially when acts
thatecological processesexplain the smallerscale ofso- ofcooperationare involved.Rappaport(I979: IOO) called
cial organization.Althoughhe cannot completelyrule attentionto the role of the sacred in concealinggroupout an ecological explanation,he clearlysuggeststhata advantageoustraitsfromreadyattackby selfishreason.
ritualsystemthatorganizesmorepeople and thus leads As the Fore experiencewith kuru illustrates,traitsdisto a greaterfrequencyof victoryin violent conflicts advantageousto groups(and to individualsin this case)
is leading to the spread of more complex ritual (pp. may sometimesbe concealed in the same way.
Knauft(i985) gives an example of an apparentgroup
I88-89).
The Tor. Significantcultural variation also existed extinctionin progress.The completelyacephalous Gebetween tribal territoriesof the Tor region (Ooster- busi were a small and declininggroupat the time ofhis
wal I961). The Tor regionis dividedinto 26 tribalterri- study.The better-organized
Bedamini,makinguse ofthe
big-manstyleofpolitical organization,were able to raid
tories,but it has 8 separatelanguages (Oosterwal I96I:
appendix).Thus, many adjacent tribes speak different Gebusi villages,but the Gebusi were unable to organize
defenseor a retaliatory
response.The boundlanguages,althoughthe most commonlanguage,thatof an effective
the Berrick,is known by membersof all tribes(Ooster- aryGebusi villagesmost exposedto Bedaminiraidswere
Oosterwal also notes these differences: in the process of assimilatingto Bedamini customs.
wal I96I:I8).
Knauft(I993a) also providesexamples of culturaldif"the threecultureareas in the Tor districtare verydisin ... kinshipterminol- ferencesamong seven culture areas along New Guintinct.... [Thereare]differences
ogy, the kinship structure,the socio-religiousaspect ea's southcoast. He describeshow theMarind-Animsysaffiliationsupportsintragroup
of culture, the way of counting, language-(dialect)- tem ofmythico-religious
and some aspects of material culture" peace and the organizationof large-scalehead-hunting
differentiations,
(p. 46). These three "cultural areas," with associated raids against distant enemies. By contrast,the Purari
kinship terminologies,are the Berrick,the Ittik and head-huntamong themselvesand are decliningrelative
Mander,and the Segarand Naidjbeedj.Tribesin "transi- to theirneighbors.The existenceof considerablevariational zones" have elements of all threeculturalareas, tion at the scale of languagegroupssuggestsa considerAlthoughthisvariand thereis variationwithineach area (pp. I49-74). The able timedepthforthese differences.
terminologyof the Berricktribeemphasizes the age cri- ation occursamonglargergroupsthanwe are concerned
terion (e.g., MoElSi is terminologicallydistinguished with here,it does show that variationin sociopolitical
fromMoYoSi) but oftenignoresthe generationalcrite- organizationencoded in mythand religionhas a strong
rion (e.g., MoBr and SiSo call each other by the same effecton groupsuccess.
It is also importantthat culturaldifferences
between
term).The terminologyof the second cultural area ig-
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SOLTIS,
BOYD,
AND
RICHERSON
Group Selection: An Empirical Test I 48I
forthe operation MODEL ASSUMPTIONS
groupspersiston time scales sufficient
of group selection. Althoughthere is variation among
The data fromNew Guinea providesome qualifiedsuplocal groupsin New Guinea, thereare no data bearing
forthe model of groupselection describedabove.
port
on the question of how long that variation persists.
I. Group disruptionand dispersal are common. ExHowever,thereis ample evidenceforthe long-termpertinctionrates per generationrange from2% to 3I%,
sistence of cultural differencesamong largergroupsin
with
a medianof IO.4% in the fiveareas forwhich quanothercultureareas. Forexample,conceptssuch as mana
titative
data are available, and the frequentmentionof
and tabu typifypolitical culturethroughoutPolynesia
elsewheresuggeststhatthese ratesare repreextinction
despite the fact that these societies have been isolated
sentative.
fromeach otherformore than i,ooo years (KirchI984).
2. New groups are usually formedby fission of exEgerton(I97I) documents the existence of important
isting
groups.The detailed picturefromthe Mae Enga
amongfourtribalgroupslivingin two differdifferences
Mendi is supportedby anecdotal evidencefrom
and
the
ent typesof environment,includingtwo tribesbelongother
We are not aware of any ethnoethnographies.
ing to the Bantu and two to the Kalenjin language
graphic
report
from
New
Guinea in which colonists of
groups,which have been separated for thousands of
are
drawn
from
new
land
multiple
groups.
years. He notes that tribal historyis more important
3.
There
is
variation
local
among
groups,but it is unenvironmentalcircumstancesin exthancontemporary
whether
this
variation
known
persists
long enough to
plainingmost of the variationin attitudesand values
and
be
to
selection
this variation
subject
whether
group
measuredin his data. The roots of the 38 languages of
is
for
the
differential
extinction
or proliferaresponsible
WesternAmericanIndiansgo back 6,500 years,and cultion
of
groups.
tural differencesamong close neighborswith different
culturalhistoryhave persistedforlong periods (Jorgensen I980:IO9). Belgium is dividedby a stable linguistic RATES OF CHANGE
boundary,with a Flemish North and a Walloon South
(van den BergheI98I); despite the factthat thereis no The New Guinea data on extinctionrates allow us to
has per- estimatethe maximumrate of culturalchangethat can
topographicalseparation,the linguisticfrontier
sisted for2,ooo years. Such examples fromarchaeology resultfromculturalgroupselection. For a given group
and historycan be multiplied at will. While they do extinctionrate,the rate of cultural change depends on
can persistat smaller the fractionof group extinctionsthat are the result of
not provethatculturaldifferences
among groups.Ifmost exscales as requiredby the model, theyindicate that this heritableculturaldifferences
tinctionsare due to nonheritableenvironmentaldifferassumptionis plausible.
ences (e.g.,some groupshave poorland) or bad luck (e.g.,
some groupsare decimated by natural disasters),then
group selection will lead to relativelyslow change. If
Discussion
most extinctionsare due to heritabledifferences(e.g.,
Cultural group selection can explain the evolution of some groupshave a more effectivesystemof resolving
behaviors and institutionsin human internaldisputes),then groupselection can cause rapid
group-functional
societies iftwo conditionsare met: First,theremust be cultural change. The rate of cultural change will also
some mechanism that preservesbetween-groupvaria- dependon the numberofdifferent,
independentcultural
tion so thatgroupselection can operate.The model de- characteristicsaffectinggroup extinction rates. The
scribed above provides one such mechanism, and we more different
attributes,the more slowly will any sinhave here tested several of the model's basic assump- gle attributerespondto selection among groups.By astions against the ethnographicrecord to determineif sumingthatall extinctionsresultfroma singleheritable
those assumptions are empirically realistic. Second, culturaldifference(or tightlylinked complex of differrapidto explain ob- ences) betweengroups,we can calculate the maximum
groupselectionmust be sufficiently
servedpatternsof culturalchange.The data fromPapua rate of culturalchange.
Such an estimatesuggeststhatgroupselectionis unNew Guinea and Irian Jayaallow us to estimate the
maximum rate of adaptation throughgroup selection. likelyto lead to significantculturalchangein less than
Thus, we can estimatea minimumtimeperiodin which 500 to i,ooo years. The length of time it takes a rare
the group-selectionprocess can give rise to group-level culturalattributeto replacea commonculturalattribute
adaptations.Cultural changes which have occurredon is one usefulmeasureofthe rateofculturalchange.Supa longertime scale are possiblythe resultofgroupselec- pose thatinitiallya favorabletraitis common in a fraction; culturalchangeswhich have occurredon a shorter tion qo of the groupsin a region.Then the numberof
time scale are unlikely to have resulteddirectlyfrom generations(t) necessaryforit to become common in a
groupselection,but theymay be its indirectresult.For fractionqt ofthe groupscan be estimated(see appendix).
example,culturalgroupselectionmay lead to the evolu- The time necessaryfordifferent
parametersis given in
tion of propertyrights,which lead to efficientalloca- table 3. If we take the median extinctionrate as repretions of resources,or of political institutionsthat lead sentative, these results suggest that group selection
to group-beneficial
decisions.
could cause the replacementof one culturalvariantby
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482
TABLE
CURRENT
ANTHROPOLOGY
Volume 36, Number 3, June1995
3
MinimumNumber of GenerationsNecessary
to Change the Fractionof Groupsin Whicha
Favorable TraitIs Common Assuminga Particular
ExtinctionRate
ExtinctionRate
InitialFraction FinalFraction
FavorableTrait FavorableTrait i.6%
IO.4%
I7.9%
3I%
I92
570
40.0
83.7
22.3
46.6
i i.8
24.8
O.I
O.OI
0.9
0.99
rateswerechosenas follows:i.6% (forthe
Extinction
Maring)is thelowestestimate,IO.4% is themedianextinction
rate,I7.9% (fortheMae Enga)is theestimatebasedon thebest
is thehighestestimate.
data,and 3I% (fortheFore/Usufura)
NOTE:
a second,morefavorablevariantin about40 generations,
or roughlyi,ooo years. If we take the extinctionrate
calculatedusingthe best data,thosefromthe Mae Enga,
this time is cut roughlyin half.These calculations assume thatcolonizinggroupsare selectedat randomfrom
the population. If groupproliferationis as selective as
groupextinction,then the time is again cut in half,reducingthe substitutiontime (based on the median extinctionrate),once again, fromi,ooo to 500 years.Not
all extinctionsand new group formationsresult from
heritable cultural differences.Since the New Guinea
ethnographicdata are not sufficientto estimatethe extentto whichculturalvariationinfluencesgroupextinctions,it is notpossible to make an estimateofthe actual
strengthofgroupselectionin New Guinea. Ifsuch estimates were possible, we expect that theywould show
thatactual rates are considerablybelow the maximum.
The maximum rate is neverthelessuseful as an upper
bound on the kinds of evolutionaryeventsthatcultural
groupselectionmightexplain.
Our estimateof the maximumrateof adaptationsuggeststhatgroupselectionis too slow to account forthe
manycases of culturalchangewhich occur in less than
500 to i,000 years.For example,accordingto Feil (I987)
the arrivalof the sweet potato in the highlandsof New
Guinea sometimein the i8th centuryled to many importantculturalchanges.The introductionof the horse
to the Great Plains of North America in the I500S led
to the evolution of the culture complex of the Plains
Indians in less than 300 years.If the rates of groupextinctionestimatedforNew Guinea are representative
of small-scale societies, culturalchanges such as these
terms. There
cannot be explained in group-functional
has not been enough time forgroup selection to have
drivena singleculturalattributeto fixation,even ifthat
attributehad a strongeffecton groupsurvival.Processes
based on individual decisions are likely to account for
such episodes of rapid evolution (see Smith and Win-
terhalderi992, Boydand RichersonI985). Such pro-
outcomesexcept
cesses will not lead to group-functional
in certainspecial circumstances(see n. 2). It is possible
that situations in which a trait or trait complex that
increases the scale of cooperationis spreadingsuch as
the one Barthposits forthe Faiwolmin do show rapid
culturalgroupselectionin progress.If the arrivalof the
sweet potato a few centuriesago did providethe subsistence basis forlargerand more complex societies, we
mightexpect to observegroupselection in the earlyto
middlestagesofthe spreadofnewlyadvantageousforms
of social organization(Golson and Gardner i990, Feil
I987).
These resultsalso suggestthatgroupselectioncannot
many differentasjustifythe practice of interpreting
A givenextinction
pectsofa cultureas group-beneficial.
unrerate will lead to slower change if many different,
lated aspects of the culture affectgroupsurvival.Suppose that both beliefsabout food consumptionand beliefs about spatial organizationaffectgroup survival.
Then, unless each extinctionoccursin a groupin which
both deleterious beliefs about food consumption and
deleteriousbeliefs about spatial organizationare common, some extinctionshave no effecton the fraction
of groupswith deleteriousbeliefsabout food,and some
extinctionshave no effecton the fractionofgroupswith
deleteriousbeliefs about spatial organization.Thus a
givennumberof extinctionsmust lead to slowerevolution of each characterthan would be the case if only
one of the charactersaffectedgroup survival.If group
selection can cause the substitutionof a single traitin
500 to i,ooo years,the rate formany traitswill be substantiallylonger.We know fromlinguisticand archaeologicalevidencethatrelatedculturalgroupswhichdiffer
in many culturalattributeshave oftendivergedfroma
single ancestralgroup in the past few thousand years.
Thus, therehas not been enough time forgroupselection to have producedthe many attributesthat distinguish one culturefromanother.
It is importantto understandthat slow does not necessarilymean weak. When individual decision making
is in oppositionto groupfunctionin everygroup,then
the relativelyslow group-selectionprocess will be too
behaviors.But when soweak to favorgroup-functional
cial interactionresultsin manyalternativestable social
arrangements,then individual decision making maintains differences
amonggroups.Iftheresultingvariation
is linkedto groupfitness,thengroupselectionwill proceed. For example,considerthe responseto an environmental change such as the openingof New Guinea to
tradewith Europeans.Initially,changesin the costs and
benefitsofalternativebeliefsand values will cause rapid
cultural change, soon leading to a new sociopolitical
equilibriumin each culture.But ifthereare manyalternative equilibria, the nature of each new equilibrium
may depend on existingnorms and values. As long as
affectgroupsurvival,selection
the resultingdifferences
among groupswill continue. Over a millenniumor so,
New Guinea societies with a betterpolitical adaptation
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SOLTIS,
BOYD,
AND
RICHERSON
Group Selection: An Empirical Test | 483
to worldcontactwill replacethosewitha pooreradapta- taken by Europeancolonists (Kelly I985). In state-level
tion.
societies,we have to allow forinternalgroupselection
Thus it followsthat these resultsdo not precludein- via the extinctionand proliferationof subgroups,such
terpretingsome aspects of contemporarycultures in as rulingclasses, interestgroups,firms,and the like, as
termsof theirbenefitto the group.The model demon- well as selection among states themselves(Hannan and
stratesthat under the rightconditionsgroup selection Freemani989). Some economistshave consideredbusican be an importantprocess, and the data fromNew ness failureand proliferationrates sufficientto drive
Guinea suggestthatsome ofthese conditionsare empir- groupselectionoftheseunits (Alchian I950, Nelson and
ically realistic.The data also suggestthat the rates of Winteri982). The developmentof collective decisiongroupextinctionare highenoughto cause a small num- makinginstitutionslike bureaucraciesand legislatures
ber of traitswith substantialeffectson groupwelfareto may permit group-functional
behaviors to be deliberevolve on time scales that characterizesome aspects of ately adopted by state-levelsocieties. These processes
cultural change. Group selection cannot explain why mightact at a much fasterrate than we have estimated
the many details of Enga culturedifferfromthe many on the basis of tribalinstitutions.
detailsofMaringculture.It mightexplain the existence
In conclusion,these data suggestthatgroupselection
of geographicallywidespreadpracticesthat allow large- cannotexplainrapidculturalchangeor the manydifferscale social organizationin the New Guinea highlands, ences between related cultures. However, they also
practicesthat evolved along with,and perhapsallowed, show thatgroupselection,perhapsin concertwithother
the transitionfromband-scale societies to the larger- processes,is a plausible mechanismforthe evolutionof
scale societies that exist today.
widespreadattributesof human societies over the long
Culturalgroupselectionprovidesa potentiallyaccept- run.
able explanationforthe increasein scale of sociopolitical organizationin human prehistoryand historyprecisely because it is so slow. Scholars convinced of the Appendix:Time forTrait Substitution
overwhelmingpower of individual-levelprocesseshave
real difficultyin explainingslow, long-termhistorical Assume that thereare two culturalvariants-deleterichange. Anatomically modern humans appear in the ous and advantageous.Each is at a local equilibriumunprocesses.Groups are
fossil record about go,ooo years ago, yet there is no derthe influenceofwithin-group
evidence forsymbolicallymarked boundaries(perhaps connectedby the mixing of individuals,and there are
*indicativeof a significantsociopolitical unit encom- many such groups.Groups in which the advantageous
passing an "ethnic" group of some hundredsto a few variantis common nevergo extinct.A fractione of the
thousand individuals) before about 35,000 years ago groupsin which the deleteriousvariantis common suf(Mellars and StringerI989). The evolution of simple feran extinctioneach generation.The dynamicsof this
states fromfood-producing
tribal societies took about systemare quite complicatedbecause the frequencyof
s,ooo years,and thatofthe modernindustrialstatetook advantageousvariantswithin subpopulationsin which
another s,ooo. Evolutionaryprocesses which lead to that variantis common depends,to a small degree,on
change on io- or ioo-year time scales cannot explain the frequenciesof both variantsin the population as a
such slow change unless theyare drivenby some envi- whole. However, if both variants are in local equilibronmentalfactorwhich changes on longertime scales. rium, even when there is only a single population in
In contrast,the more or less steadilyprogressivetrajec- which they are common, then it is roughlycorrectto
toryofincreasingscale ofsociopoliticalcomplexityover regardthe subpopulationsas individualsand use formuthe past fewtens of thousandsofyearsindeed is consis- las frompopulation genetics (see Boyd and Richerson
tent with adaptation by a relativelyslow process of iggoa fora fullertreatment).Then, if the advantageous
trait is common in a fractionq of the groups in the
groupselection.
These resultsshould be interpreted
with caution.It is region,afterone generation
importantto rememberthatwe have estimateda maxi,=
~q
mum rate of change forgroupselection on the basis of
- e) + q
(I - q)
the assumptionsthat observeddifferences
among local
groupsare heritableand thattheyare persistent.Unless and the frequencyaftert generationis
both assumptionsare satisfied,groupselection will be
qo
less importantthanourresultsindicate.It is also imporqt_(
tant to keep in mind that we have studied only one
e)t + qo
(I qo)(iformofgroupselection-competition amongsmall, cul- Solvingthis fort yields
turallyheterogeneousgroups. Other plausible groupselectionprocessesmightlead to morerapidchange.For
In (qo(I- qt))
example, one cultural region may encroach upon an(I - qo) qt
other along a frontier,constantlycapturingadditional
In(i - e)
land and graduallyexpandingits domain. The Nuer and
Dinka formedsuch a systembeforetheywerebothover- which was used to generatetable 3.
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484
CURRENT
ANTHROPOLOGY
Volume 36, Number 3, June1995
Comments
C. R. HALLPIKE
McMaster University,
DepartmentofAnthropology,
Hamilton, Ont., Canada L8S 4L9. 2 XII 94
Soltis, Boyd,and Richersonare to be congratulatedon
providing,forthe firsttime,a solid empiricaltest of the
social traitsare theresult
beliefthatfunctional/adaptive
ofsome processofgroupselection.Their demonstration
of the extremeslowness of such a process effectively
places it on the marginof any evolutionarytheoryof
human society.Endogenousprocesses (such as changes
in the resource base) and diffusionare obviously far
more powerfulexplanations,for which there is also
abundantevidence.
It is worthnoting,however,that this is yet another
demonstrationof the basic implausibilityof the whole
style of explaining social
functionalist/adaptationist
evolution,because some model ofgroupselectionis the
onlypossible theorythat has ever been producedto extraits
plain why we should expect functional/adaptive
as the norm in human society. (I am excluding, of
course, those traits which are conscious responses to
obvious problems,such as bandagesforwounds.)
Once this selectionistmodel has been rejected,the
only solution to the functionalistdilemma is the one
thatI haveadvocated(Hallpikei986:86-i22),
whichis
that thereneverwas such a problemin the firstplace.
This is because in small-scale societies with simple
technologiesthereare many ways of organizingsocial
and within
relationsand ofadaptingto the environment,
verybroadlimits theywill all work.This beingso, it is
futile to tryto explain any of them as some kind of
optimal strategyforanything,and the reason theyexist
will be a matterofhistoricalcontingencies.To call some
culturalfeatureadaptive will only mean that it must
meet some minimalcriteriaofpracticaladequacy.Func"explanations"always dependon the
tionalist/adaptive
post hoc fallacythatbecause somethingis beneficialto
someone in some way (which is never hard to prove)
this must be why it has survived.
One cannotagree,however,withthe authors'attempt
to salvage somethingofthe groupselectiontheorywhen
they say, "Cultural group selection provides a potentially acceptable explanation for the increase in scale
of sociopolitical organizationin human prehistoryand
historypreciselybecause it is so slow." In thefirstplace,
endogenousprocesses and diffusionare quite adequate
explanationsforthe slow increasein sociopoliticalcomplexityto which theyreferand thereforeprecludeany
need to call upon groupselection.But some ofthe most
obvious examplesofincreasein social scale, such as the
Roman Empireor the IndustrialRevolution,have been
extremelyrapid, and these cases obviously cannot be
explained by group selection. To move to the level of
subgroups,such as interestgroupsor business firms,as
the authorscontemplatedoing,is no solution because
such groupsare themselvesonly componentsof wider
social processes. If the demise of subgroupsis to be
takenas a valid instanceofgroupselection,why should
it be limitedto firmsthat go bankruptand not also include couples who divorce?The obvious absurdityof
this example demonstratesthatwhen we are discussing
group selection we must confine ourselves to groups
which are self-sufficient
communities.
MASAKADO
KAWATA
Departmentof Biology,Faculty of Education,
Shizuoka University,Ohya 836, Shizuoka 422, Japan
([email protected]). 9 XII 94
Soltis, Boyd, and Richerson's article is an important
analysis that shows when and to what extentcultural
groupselectionplays a role in culturalevolution.It is a
significantcontributionto the fieldsof both anthropology and evolutionarybiology. Although they do not
denya role forgroupselection over a long time period,
theysuggestthatit cannot explain culturalchangeover
a shorterperiod.I do not object to the main framework
of theiranalysis,but I would like to add some pointsto
theirdiscussion.
The most importantassumption of group selection
in grouppropertiescauses difmay be that a difference
ferencesin group survival and extinction.Soltis et al.
recognizethis assumptionand discuss how culturaldifferencesin groupsrelate to differences
in groupfitness.
It is important,however,to distinguishgroupselection
fromgroup drift.If there is cultural variation among
groups,some cultural traits may change in their frequency by groupextinctionor by new groupformation
even ifthe traitsdo not relateto the causes ofextinction
and formationof the group(i.e., are neutral).This process is similarto randomgeneticdriftand therefore
can
be called groupdrift(TurnerI988). Both processes are
potentialcauses of changein the frequenciesofcultural
traits.
Anotherimportantpoint that Soltis et al. do not discuss is the distinctionbetweengrouppropertiesand individualproperties.Political organization,spatial distribution within tribes,and ritual practicesare examples
of groupproperties.In contrast,foodhabits adoptedby
individualsare examplesofindividualproperties.Ifindividuals can choose a foodhabitirrespectiveofgroupculturalforcessuch as the religiouspreference
ofthegroup,
an increase in the numberof individuals adoptingthe
foodhabitmay cause the extinctionofthe group.But in
this case the groupextinctionis caused merelyby the
sum of individualdeaths caused by the propertiesof individuals-individual foodhabits.Thus, thisis an example not of groupselection but of individualselection.
Soltis et al. suggestthat culturalevolution by group
selectionmay be a slow processon the basis oftheirlow
estimatesof extinctionrates. But thereis anothertype
ofgroupselectionthatdoes not requiregroupextinction.
In evolutionarybiology,groupselectionhas sometimes
been classified into two types: interdemicand intra-
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SOLTIS,
BOYD,
AND
demic (Wilson I983). The groupselection that Soltis et
al. discuss is analogous to interdemicgroupselection(or
selection).In intrademicgroupselection,also
intergroup
called traitgroup selection, a differencein the growth
ratesofgroupscauses changesin gene frequencies(Wilson I980). In this typeof groupselection,groupproperties should affect differentialgrowth rates among
groups. Afterthe dispersal of individuals, individual
propertiesthat produce the group propertiesaffecting
growthrates increase in frequencyin a global population.An analogous processmay occur in culturalevolution (here this is called cultural traitgroup selection).
Forinstance,a grouppropertysuch as a political organization may cause an increase in the size of the group.
Consequently,the groupwill produce more emigrants
to, or will increase the chance of contact with, other
groups.This, in turn,will increase the probabilityof
culturaltransmissionto those groups.
Sober(I984) distinguishedtwo conceptsin naturalselection: propertiesforselection (i.e., propertiescausing
selection)and objects of selection (i.e., objects that can
reproduceand die). For intergroupselection,properties
forselection are grouppropertiesand the objects of selection are the groups.For traitgroupselection,in contrast,propertiesfor selection are group propertiesbut
for
the objectsof selectionare individuals.Accordingly,
culturaltraitgroupselectionas fortraitgroupselection,
propertiesforselection are groupculturaltraitsand objects of selection are individuals. Cultural traitgroup
selection does not require the extinctionof groups,as
does intergroupcultural selection, and thus cultural
change by this process may be more rapid.In addition,
thisprocesscan be examinedbyusing contextualanalysis (Heisler and Damuth I987) of present-daysocial
groups.Intergroupselection and culturaltraitgroupselection may affectthe maintenanceand change in frequency of cultural traitsover a short time period,althoughthe replacementof one culturaltraitby another
over such a shortperiod cannot be explained by these
groupselections,as Soltis et al. have suggested.
RICHERSON
Group Selection: An Empirical Test | 485
membersand will be extinctwithina fewyears(Gebusi
do not admitchangesin clan affiliationsuch as cumulative patrifiliation).Two other Gebusi clans may have
become extinct since I 940 (no survivorsare known,
thoughit is possible thatsome survivedoutsidemy survey of clan genealogies),and several otherclans are in
dangerof extinction.I estimate the Gebusi extinction
rate to have been equivalent to 3-4 per 22 clans per 50
years, yieldinga 25-yeargroup extinctionrate of between 6.8% and 9.I%. Gebusi clan extinctionis due
to naturaldemise and the killingof suspectedsorcerers
ratherthan the impact of introduceddiseases.
Anothercase not cited by Soltis et al. is population
replacementin theIlaga ValleyofIrianJayadocumented
in detailbyLarson(I986). FromaboutI9I0 to I96I, the
indigenousDamal populationwas subjectedto a series
of I4 majorwars;it declinedapproximately
i5%, from
I,300 to I,I00. Overthistimeperiod,
theintrusive
Ilaga
Dani populationincreasedat least sixfold,from600-700
to 4,I00. However, significantDamal groupswere displaced outsidethe Ilaga Valley and formedbitribalcommunities and political confederacieswith Dani. This
raises a long-standingproblemthat Soltis et al. skirt:
Whatis a "group"?Presidentialgroups,politicalgroups,
culturallyascribedkinshipgroups,and groupsof biogenetic kin are nonisomorphic.Under certainconditions,
culturalgroupscan changetheiridentityand theircomposition incrementallyratherthan simply "surviving"
or "becomingextinct."This is theoreticallyimportant
because, like most evolutionary modeling analyses,
Soltis et al.'s is highly dependentupon its initial assumptionsand definitions;a small changein the definition of "group" can produce a substantiallydifferent
outcome.
The kindsofsedentaryand relativelydiscreteresidential kin groupsthat Soltis et al.'s analysis seems to assume forNew Guinea are relativelylate in evolutionary
termsand not a good model forassessingthe fullpotential of groupselection over the longercourse of human
development
(seeKnauftI989,
I99I,
I993b,
I994,
n.d.).
Most dispersedforagersexhibitedfluidresidentialcomposition facilitatedby numerous fictive kin, classifiM. KNAUFT
BRUCE
catory clan, totemic, and affinalrelationships.These
EmoryUniversity,
DepartmentofAnthropology,
arrangementswere consistent with the social and
residentialflexibilityneeded forthe full exploitationof
Atlanta, Ga. 30322, U.S.A. 28 XI 94
resourcesthat were dispersed,patchy,and hard to deUnits of selection above the individual (and below the fend.Given the small population size and low density
would not appearto lessen
gene) are receivingincreasingscholarlyattention(see ofsuch bands,thisflexibility
Wilson and Sober I994); it is appropriatethat groupse- the importanceof collectiveselection (e.g.,see Rogers's
lectionbe seriouslyreconsideredin human culturalevo- [I990:408]
thatgroupselectionvia selective
suggestion
lution. The rate of group extinctionthat Soltis, Boyd, emigrationis facilitatedby mobilityratherthan by isoand Richerson assess for precolonial interior New lation of local groups). Importantfor Soltis et al.'s
Guinea seems appropriate.A substantial review indi- model, dispersedforagersfromtwo geographicallydiscates that the rate of indigenouswarfare,local killing, tant regionsseldom fuse or interpenetrate
to inhabit a
and groupturnoverwas high in many areas of interior new locale.
New Guinea (KnauftI990, i992; see also Healey I985:
Concerningthe maintenance of variation,Soltis et
on Maring).Ge- al.'s notion of groupselection appearsto adopt the conII -I4 with more detailed information
busi can be added to Soltis et al.'s specificcases (Knauft servativeassumptionthat all traitswill spontaneously
I985, I987); I of the 22 clans has become extinct in diffusebetween groupsunder conditionsof social conthe past 40 years,and a second has no remainingmale tact.While thismay be the case formatingbehaviorand
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486 1 CURRENT
ANTHROPOLOGY
Volume 36, Number 3, June1995
gene pool dispersionamong nonhumanpopulations,it biologistsbecause of the possibility,now thoughtto be
is farfromuniformfortraitsinfluencedby human cul- small, that it mightlead to the evolution of altruistic
ture-even among flexiblyintertwinedgroups.Diver- behaviour.It seems to be a common assumption that
sityin behaviorand beliefcan be preservedin thefaceof cultural group selection could explain self-sacrificial
social contactand in the absence of ethnicpolarization traits(Campbell I975). Yet even if culturalgroupselecbetween "groups." Among Gebusi subgroups,for in- tion is plausible, thereare at least two good reasons to
stance,chains of divergencein a numberof residential, be suspicious ofthis assumption.First,how manytraits
bodily,aesthetic,and discursivepracticeswere evident are likelyto impose a fitnesscost at an individuallevel
despitehighlyprosocial affiliationsand common tribal and still be able to increasethe persistenceor proliferaidentityacross the gradients.The strongneed to pre- tion of groups?For a population to bud offinto a new
serve variation in orderfor selection to operate (Dar- group requires sufficientindividuals to people it, but
win's dilemmain ignoranceofMendelian genetics)does self-sacrificial
traitsby definitionreduceindividualvianot seem to be as much ofa problemforgroupselection bility.Clearlythe rateofgroupproliferation
is not indein human populationsas Soltis et al.'s conservativeas- pendentof natural selection. Does this mean that the
sumptionsimply.The complementto this microdiffer- only stable group-functional
traitswill be those also faentiationofcultureis the distinctiveculturalpropensity voredbynaturalselection?Can culturalgroupselection
constantlyto generateand producenew variation.The onlyoperatein conjunctionwithnaturalselection?Secgreatcuriosity,neoteny,and cognitivedevelopmentof ond,unless the pressureto conformis intense,individupon which sponta- ual learningwill reduce the frequencyof maladaptive
humans providethe infrastructure
neous culturalelaborationofhumanideas and behaviors cultural traits. The authors mention the cannibalistic behaviourof the Fore, which facilitatedthe spread
takes place on a continuingbasis.
These commentssuggestthat groupselection among of kuru and almost certainlyincreased the chances of
humans may have been more importantthan Soltis et groupextinction.It is interestingthat the Fore did not
realistic eat people who died of dysenteryor leprosy,since they
al.'s assumptionsallow. More ethnographically
assumptionsabout human groupsand about the sponta- had learned that these conditionswere infectious,but
neous generationand maintenanceof culturalvariation other causes of death did not deter this practice (Linare likelyto increasethe importanceof collectiveadap- denbaumI979). This suggestsa roleforindividuallearning in the dynamic,erodingculturalvariantswhich are
tationsin the considerationof human evolution.
obviouslymaladaptive.We mighthypothesizethat the
onlymaladaptivevariantsthatcan be stablytransmitted
across generationswill be those that pass the selective
N. LALAND
KEVIN
filterofindividualexperience.Perhapsthe authorscould
Sub-DepartmentofAnimal Behavior, Universityof
clarifythe conditionsunder which they would expect
Cambridge,Madingley,CambridgeCB3 8AA, United
self-sacrificial
behaviourto evolve by this means.
Kingdom.7 XII 94
I wonderwhetherSoltis et al. maybe underestimating
Gene-culturecoevolutionarytheoryis undoubtedlya the role of cultural group selection in other respects.
valuable set of tools with which human scientistscan They focus exclusively on extinctionsas the eliminaexplore the interactionbetween genetic and cultural tion of populations of individuals rather than popuprocesses.Over the past two decades, Boyd and Richer- lations of cultural variants. If some cultural variants
ofthe developmentofthis become extinct simply because the idea dies out or
son have been at theforefront
theory.I considertheirtheoreticalinvestigationof the individuals choose not to propagate it, then the frefeasibilityand consequences of culturalgroupselection quency of groups with the cultural variant will have
to be one of the most importantachievementsof the been reducedjust the same as ifthepopulationhad been
field in recent years. The significanceof this paper is destroyed.Considerationofbothtypesofextinctionwill
that,togetherwith Soltis, theyhave set out to test this increasetheratesofchangeofculturalvariantsbygroup
body of mathematicaltheory,using data fromsocieties selection.Secondly,a culturaltraitclearlydoes not have
in New Guinea. The authorsare to be commendedfor to affectthe probabilityof grouppropagationor extincto integratetheoreticaland empiricalfind- tion to spreadby groupselection,althoughgroupselectheirefforts
ings and illustratehow theorycan spawn empiricalin- tion is a weaker forcewhen this is the case. Nonethevestigation.I hope thatthis paperwill act as a bridgeto less, the extinctionof a population is a very dramatic
assist those motivatedto tackle Boyd and Richerson's eventcomparedwiththe deathofan individual,the loss
moretechnicalpapersand will stimulateinterestin the of an allele, or the exterminationof an idea, since the
workofotherpractitionersin the field,notablyCavalli- frequencyof a largenumberof traitsin a largenumber
Sforzaand Feldman. At the veryleast, it has informed ofindividualsis affected.Considerthe fateof "neutral"
the debate on the feasibilityof culturalgroupselection. culturalvariantsthat do not affectindividualviability:
Now thatSoltis,Boyd,and Richersonhave established While natural selection does not change the frequency
thatculturalgroupselectionis not implausible,perhaps ofneutraltraitsunless theyare stronglyassociated with
theycould reconsiderwhetherculturalgroupselection selected variation, in contrast group selection will
behaviour.The mecha- changethe frequencyof all neutralvariants.This raises
can reallyexplain self-sacrificial
nism of groupselection was of interestto evolutionary the possibilitythat if the authorswere simultaneously
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SOLTIS,
BOYD,
AND
to consider the dynamics of a number of traits (i.e.,
multilocus models at a culturallevel), group selection
could be foundto be a more significantagent.
JAMES G. PEOPLES
Ohio
Departmentof Sociology/Anthropology,
Wesleyan University,Delaware, Ohio 43015, U.S.A.
29 XI 94
Soltis,Boyd,and Richersonhave givenus anotherpaper
thatextendsand applies some of the evolutionarymodels formulatedin the past fifteenyears by Boyd and
Richerson.In theirpioneeringapproach,the frequency
ofa givenculturalfeaturein a humangroupresultsfrom
the operationof certainevolutionaryforcesthat affect
rates of intergenerationaltransmission(discussed and
modeledin Boydand RichersonI985). The factthatthe
mechanism of cultural transmissionis social learning
ratherthanbiologicalreproductionimbues culturewith
propertiesthat make humans unique. Two of the most
importantof these propertiesare that (i) humans cooperate on a much largerscale than otherorganismsand
(2) groupselectionis potentiallya morepowerfulevolutionaryforcein humanbehaviorthanin otherorganisms
(Boydand RichersonI98 2, I987, I9gob).
The presentpaper attemptsto determinewhetherestimated rates of group extinctionamong well-studied
high forcooperaNew Guinea peoples are sufficiently
tion in warfareto be fixedby a process of groupselection. A more generalsuggestionis that groupselection
is more likely to be responsibleforchanges over very
long time spans than over shortperiods.
The authorsperforma valuable servicein pullingtogetheravailable data on groupextinctionrates forprecontactNew Guinea. I agreewith the essentialsoftheir
model of how group selection might work in human
populations, and I applaud their effortto determine
whetherthe conditionsforthe model are met by real
human groups.My main reservationshave to do with
(i) the implicationsoftheirfindingson groupextinction
ratesand (2) the conclusion thatgroupselectioncan explain onlyverylong-termculturalchanges.
i. The data presentedon extinctionrates are critical
because groupselection requiresrelativelyhighrates of
biological or social extinction.In New Guinea, the authorsdoubtthatgroupextinctionratesare highenough
forcooperationin warfareto have been fixedby group
selection.Their skepticismarises fromtheirunspoken
assumptionthat observedextinctionrates apply to the
past as well as the present-that is, thatrateshave been
roughlyconstantover long time periods.But if extinctionrateswerehigherduringprevioushistoricalperiods,
thenpast groupselectioncould have fixedhighlevels of
cooperationamong present-daygroups such that they
seldom go extinct-that is, overlong time spans groups
approachedequilibriumwith one another,resultingin
low extinctionin the present.Indeed,underconditions
oforganizedintergroupconflict,intuitionas well as the
New Guinea ethnographicrecordsuggeststhatshortpe-
RICHERSON
Group Selection: An Empirical Test | 487
riodsof violentwarfarealternatewith longerperiodsof
uneasypeace. To determinewhetherextinctionratesare
sufficiently
highwe need data collected duringtimes of
greathostility,not data gatheredduringpeacefulintervals and not data collectedafterthe groupselectionprocess may have run its course. Such data are difficultto
findand collect. My firstcomment,then,is that information about recent extinctionrates is an inadequate
basis on which to judgewhethercooperationcould have
been fixedby historicalgroupselection.
2. The article's conclusion suggeststhat,under realworldconditions,groupselectionoperatesso slowlyrelative to other processes that it cannot explain most
short-termcultural changes. However, this suggestion
is not compelling,forthe authors overgeneralizetheir
findings:one cannot use data on extinctionrates from
one part of the world to conclude that groupselection
is unimportantelsewhere.We just do not know whether
groupextinctionratesin otherplaces and timesare comparable to those given forNew Guinea. Lacking such
we cannotconclude (althoughcertainlywe
information,
can "suggest")thatgroupselection is a weak force.
In my view, groupselection can be a powerfulevolutionaryforce to explain a given cultural feature(s)X
when all of the followingconditionsexist: (i) coequal
groupscompete over resources,with the losers becoming socially or biologicallyextinct;(2) competitivesuccess againstothergroupsincreaseswith increasedcooperation;(3) the benefitsof cooperationare public goods;
(4) the willingnessto cooperate depends upon cultural
featureX, but actorsare unaware of the connectionbetween featureX and their cooperative behavior; and
(5) competitivesuccess leads eitherto territorial
expansion throughsubdivisionor to growthof the successful
population(s),which replicatesits culturalfeaturesover
long time spans. "Coequal" in condition i means that
no grouphas such a strongcompetitiveadvantagethat
it can reliablyand regularlyoverwhelmits competitors.
"Socially extinct"means thatthe groupdisintegratesas
an organizationand the membersdisperse.Condition2
simply says that success varies with the ability of a
groupto induce its membersto cooperate(all else equal).
Condition3 is necessarybecause if noncooperatorscan
be excluded fromacquiringbenefits,then selfishrationalityor tit-for-tat
may be sufficient
to lead to sufficient
cooperationand selectionbetweengroupswill not operate. Condition4 requiresthatgroupmembersnot recognize that theirwillingnessto cooperateis partlydue to
the existenceof featureX but believe thatfeatureX exists forsome otherreason(i.e.,thefunctionalistassumption).This conditionis necessarybecause otherwisethe
characteristicto be explained (featureX) is more likely
to be explainedbyplanningor some otherformofgroup
decisionmakingthanby groupselection.Condition5 is
necessaryforfeatureX to spreadamongthemetapopulation,as the articlepoints out.
These five conditions may appear restrictiveand
groupselectiontherefore
rare.However,it is reasonable
to believe thattheseconditionswill oftenapplyto many
horticulturaland pastoralpeoples engagedin organized
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488 1 CURRENT
ANTHROPOLOGY
Volume 36, Number 3, June1995
intergroupwarfare,to firmsin a competitivemarketplace, to politicalparties,to athleticteams,and to business associations. I suspect that featureX will commonly take the form of group-enactedrituals and
ceremonies,ethnic and otheremblematicsymbols,abstractethical principles,and values of loyalty.
ANDREW
P. VAYDA
Departmentof Human Ecology,Cook College,
RutgersUniversity,New Brunswick,N.J.o89o3,
U.S.A. 2I XI 94
ties,were responsibleformaintainingknowledgeof the
ritesin secrecybetweentimes of theirperformanceeverytenyearsor so. This explanationis citedand presumably accepted by Soltis et al., but theyfail to mention
thatBarth(i987:24, 26, 27) referred
also to borrowing
fromothercommunitiesas a means of fillingthe gaps
in ritual specialists' memories. As variable combinations of what is remembered,what is improvised,and
what is borrowed,ritual practicesand beliefsmay still
differamong Mountain Ok groupsat any one point in
time,but thedifferences
can hardlybe takenas evidence
of the kind of intergenerationally
persistingcultural
variations called for by Soltis, Boyd, and Richerson's
model of culturalgroupselection.
As for evidence of local-groupextinctions,I must
question the decision by Soltis et al. to considerextinction only throughwarfareand theirjustificationof this
decisionon the groundsthatsuch extinctionmay be the
common fateof declininggroups.AmongMaringthere
were,as indicatedbymyunpublisheddata on changesin
local groups,considerablymore local-groupextinctions
thanthe singleone cited by Soltis et al. frommy article
on Maringwarfare(Vayda I 97 I). These extinctionswere
not necessarily connected proximatelywith warfare;
theyoccurredexclusivelyat the lower altitudeswhere
Maringlocal groupswere,because ofenvironmentalfactors,much smaller and much more subject to deaths
frommalaria than in the higher,core areas of Maring
settlement(cf.Lowman I980 and the summaryof it in
Althoughgenerallyan admirerof Boyd and Richerson's
work on processes and mechanisms of cultural evolution (see Vayda I995), I must questionwhethertheyand
coauthorSoltis have been able to findin New Guinea
data theevidencetheyneed fora meaningfultestoftheir
model ofgroupselection.Thus, fortheircrucialassumption that biased cultural transmissionis a mechanism
among small local groups
wherebyculturaldifferences
there
aremaintaineddespitecontactsand intermarriage,
is no evidenceat all. Consistentwith this,therealso is,
as the three authors concede, no evidence fromNew
Guinea thatculturalvariationamonglocal groups"persistslong enoughto be subjectto groupselectionand ...
is responsible for differentialextinction or proliferation." And there is evidence of tradingand marriage
contacts, as well as other mechanisms,which would
have-or, at least, could have-worked againstthe long FoinandDavis I987:I3).
As I noted
My reason forreferring
here to these extinctionsis
persistenceofparticularculturaldifferences.
long ago (Vayda i966:294), the widespreadculturaluni- merelyto set the recordstraightratherthan eitherto
model.
formitywhich has impressedsome anthropologistsin supportor to oppose the Soltis-Boyd-Richerson
Read I954:6) may In line with what I have said at the beginningof this
New Guinea (e.g.,Pouwer i96i:i-2;
well have resultedfromthe tradingand intermarriage comment,I regardevidence of local-groupextinctions
which joined local groupsin diffusionchains extending as havinglittlerelevanceto theirmodel in the absence
sometimesfordistances of more than ioo miles (Pos- of evidence of long-persistentcultural differencesbeSalisbury i956:562; Vayda, Leeds, tween those groups which become extinct and those
pisil I963:337-38;
n. io). I furthernoted in the same which do not.
and Smith I96I:73
articlethatmy own studiesin the BismarckMountains
on the fringeof New Guinea's centralhighlandsindicated that even when wives comingfromothergroups DAVID SLOAN WILSON
werenativespeakersofdifferent
languagesand made no Departmentof Biological Sciences, Binghamton
substantialdirect contributionsto the cultureof their University,State Universityof New York,
husbands'groups,a resultofmarriagewithsuch women Binghamton,N.Y. I3902-6000, U.S.A. I7 XII 94
was affinalconnectionsservingas channelsforcultural
to meaimportations.Informantscited not only certain new Soltis et al. make a valuable and judicious effort
crops and tools but also certainnew rituals as having sure rates of cultural change that can be expected to
been obtained by virtue of such connections (Vayda occur at the group level. The very fact that they are
attemptingto providedata fora subject that is domiI966:295).
Other New Guinea ethnographershave described nated by speculation deservespraise. I thinkthat their
of500-I,OOO yearsis reasonable
forsomekinds
othermechanismsthatwould have workedagainstlong estimate
persistence of particular cultural differences.Barth's of culturalgroupselection but that otherkinds can ocMountain Ok case (I987) is noteworthybecause of the cur morerapidly.Thus, theirsuggestionthat"groupseseveral paragraphsdevoted to it by Soltis et al. To ex- lection cannot justifythe practiceof interpreting
many
thathe foundamongMountain Ok different
is not
aspects of a cultureas group-beneficial"
plain the differences
groupsin ritesand associated cosmologicalideas which warrantedby theiranalysis.
It is importantto distinguishbetween two kinds of
had evidentlycome froma common source, Barthreferredto guessworkand improvisationby the elderly cultural evolution. Some cultural transmissionrules
ritualexpertswho, with imperfectmnemoniccapabili- perpetuatebehaviors but do not automatically cause
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SOLTIS,
BOYD,
AND RICHERSON
themto spreadthroughthepopulation.Forexample,the
rule "Do what yourparentsdo" will simplycause traits
in the same fashto be passed fromparentsto offspring
ion as genetictraits.Whethera particulartraitincreases
in frequencywill depend on differentialbirths and
deaths and thereforewill have a time scale similar to
geneticevolution.Otherculturaltransmissionrules actuallycause traitsto spreadof theirown accord,sometimesveryrapidly.Forexample,ratsare normallyreluctantto accept novel foods(whichmay be poisonous)but
will readilydo so if theysmell the odor of the food on
anotherrat.The transmissionrule "Accept a novel food
ifyou smell it on anotherrat" can cause the acceptance
of a novel food to spread rapidlythrougha population
withouta differential
birth-and-death
process.
To understandthe second kind of transmissionrule,
we must ask how it evolved. In the rat example, it is
fairlyobvious that the transmissionrule is biologically
adaptivebecause the novel food will not be poisonous
ifanotherrathas been eatingit. Furthermore,
it is adaptiveat theindividuallevel. Rats thatfollowthe decision
thanratsin thesame popurulewill have moreoffspring
lation that do not. Thus, the decision rule probably
evolvedby individualselection.
It is conceivablethatthe second kind oftransmission
rule can also evolve by groupselection (see Wilson and
Sober I994 fora review of groupselection as it relates
to humanbehavior).Forexample,imaginethatratshave
a special alarm call that theygive when theirgroupis
challengedby another,hostile groupof rats. The alarm
call is repeatedby any ratthathears it beforerushingto
the source of the disturbance.The transmissionrule
"Call ifyou hear a call" does not increasethe fitnessof
callers relative to noncallers in the same group but
rathercauses the whole groupto mobilize quickly,increasingits fitnessrelativeto othergroups(at least as
the example is constructed).
When we observethe culturaltransmissionof alarm
calls, do we say that it evolves by cultural group selection? Obviously not, if we are looking at the hereand-now process of cultural transmission,because it
spreads within the groupin a matterof seconds. In a
moreultimatesense,however,we are justifiedin saying
that the behaviorevolved by groupselection,since the
culturaltransmissionrule that promotesit evolved by
this
groupselection.We are also justifiedin interpreting
aspect of cultureas group-beneficial.
To state the argumentmore generally,transmission
rules that cause traitsto spreadthroughthe population
can evolve by within-or between-groupselection. The
role ofgroupselectioncan be evaluatedonlyby examining the fitnessconsequences of the traits.If the transmission rule consistentlypromotestraitsthat increase
the fitnessofgroupsrelativeto othergroups,it probably
evolved by group selection (as in the alarm-callexample). If it consistentlypromotestraitsthat increase the
relativefitnessof individuals within groups(as in the
food-acceptanceexample)it probablyevolvedbywithingroupselection.
The transmissionrules that Soltis et al. considerare
Group Selection: An Empirical Test | 489
mix ofthe two kindsofculturalevolution
an interesting
discussed above. The majorityrule makes the most frequent traitsactively spread to fixationwithin groups,
but then a differentialbirth-and-death
process at the
grouplevel is requiredforadditionalspread. Five hundredyearsis remarkablyshortas differential
birth-anddeath processesgo. How long would it take a traitthat
is culturallytransmittedfromparents to offspringto
spread by a differential
birth-and-death
process at the
individual level? It is extremelyimportantfor us to
know the probableratesofongoingculturalgroupselection, and no more should be expected of the targetarticle. However,fora complete understandingof group
selection and culturalevolution we must examine the
evolution of transmissionrules that actively propel
traitsthroughpopulations.To the extentthattheypromotegroup-levelfunctionalorganization,theymaywell
have evolved by groupselection.
Reply
JOSEPH
AND
SOLTIS,
PETER
ROBERT
BOYD,
J. RICHERSON
Los Angeles, Calif., U.S.A. 2o
I
95
We are gratefulto the several commentatorswho have
suggestedusefulamendmentsto our paper. Knauftprovides an additional extinction-rateestimate fromhis
work among the Gebusi. This estimate (between6.8%
and 9.I%) provides a 2o% increase in the size of our
sample! Knauftalso makes a good point about the importanceof how groupsare defined.The size of groups,
the degreeto which theyare bounded,and theirinternal
structureare importantempiricalquestions.Our simple
model seems to be an adequate firstapproximationfor
precolonial New Guinea, but undoubtedlythings are
more complex elsewhere.As we note, the specificform
ofgroupselection thatwe modeled is only one ofmany
possible forms.
Kawata makes two usefultheoreticalsuggestions.As
he notes, we neglectedthe effectof driftat the group
level. Such a process will act wheneverextinctionand
recolonizationare randomprocessesand the numberof
groupsis small. Group drifthas two effects:it reduces
variationamong groups in the same way that genetic
driftreduces variationamong individuals,and it introduces a randomgroup-levelprocess that may oftenoppose groupselection.As a result,groupdriftwill reduce
the likelihood that group-functionalbehavior will
evolve. Groupdriftis likelyto be relativelymoreimportantthanindividualdriftbecause thenumberofcompeting groupswill typicallybe small. Kawata suggeststhat
it is importantto distinguishgroupand individualproperties.We agree.Because we focus on selection among
groups at differentlocal equilibria, all selection discussed in this paper is selection for group properties.
We also concur with Peoples's suggestionthat group-
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490
CURRENT
ANTHROPOLOGY
Volume 36, Number 3, June1995
beneficialbeliefs,if theydo occur,are most likelyto be unsuccessful firms go bankrupt (Nelson and Winter
involvedin ritual and religionbecause such beliefsare i982, Hannanand FreemanI989). In principle,
thereis
most resistantto the corrosiveeffectsof rationalcalcu- nothingabsurdabout selection amongmarriedcouples.
lation.
If marriedcouples varied culturally,if these variations
Hallpike offerstwo reasons that group selection is affectedthe divorce rate, and if these variationswere
even less importantthan our results suggest.First,he transmittedto the next generationof marriedcouples,
arguesthatgroupselection cannot account forthe long- thenselectionat thatlevel would increasethefrequency
termincreasein social complexityseen in the archaeo- of beliefsthat inhibiteddivorce.In fact,however,marlogical recordbecause there are examples of rapid in- ried couples probablydo not transmittheirparticular
crease in scale and social complexity such as the social arrangementsintact to othermarriedcouples, at
expansionofthe Roman state and the industrialrevolu- least not veryoften.
We partiallyendorse Hallpike's claim that thereare
tion. There is no doubt that particularpolities have
vastly increased in size in shortperiods of time. One manysolutionsto group-functional
problems.Such varicould add the Inca, Aztec, and Mongol empiresto Hall- ation is consistentwith the action of a slow, awkward,
pike's list, along with many otherexamples. However, historically contingentprocess like group selection.
the expansionofa particularpoliticalunit need not cor- However,his blanket dismissal of groupfunctionalism
respondto the evolution of beliefs,norms,and institu- on the basis of such evidence veryplainlygoes too far.
tions thatpermitthe increase in scale. It is likely that It is easy to cite a vast numberof well-describedcases
the cultural changes that allowed Rome to expand at in which social-organizationaldifferences
influencethe
theirneighbors'expensewere underway longbeforethe outcome of intergroupconflicts.Barth's (I98I) classic
Roman expansion and were part of a long traditionof studyof Swat is a typicalexample. The feudal Pathan
city-statepolitiesin theMediterraneanbasin. Moreover, systemcould expandat the expenseofKohistanivillagemany norms and institutionsthat spread with Roman level polities,but only to where environmentallimits
militaryand economic success did not disappearwhen permittedenough farmproductivityto supportPathan
the Roman state collapsed. Instead,theywere adopted warriorelites. Beyondthat frontier,
the Kohistanishad
by othergroupsin Europe and the Middle East and, re- an advantage,and the intercommunity
boundaryhas recombinedwith local concepts,may have providedthe mained stable forhundredsofyears.More generally,we
take it that there are three featuresof human social
basis forArab and laterEuropeanstates.
Second, Hallpike argues that selection among sub- groupsthathave to be explained: (i) Large-scalehuman
units of societies such as lineages or firmsis not group societies workwell enoughto have made us the earth's
selection because theyare embeddedin and dependent dominantanimal species in the past io,ooo years. (2)
on a largersocial system.He arguesthatwe shouldlimit Human cooperativeinstitutionsare highlyvariablefrom
communi- place to place and time to time. (3) These institutionsthe term"groupselection" to self-sufficient
ties. He attemptsto demonstratethis by the reductio even the best of them-are today veryfarfromexhibad absurdum-if group selection can apply to firms, itingadaptiveperfection.Hallpike leans on evidencefor
why not to marriedcouples? We don't care verymuch 2 and 3 in orderto discount i, but i is, it would seem
what gets labeled groupselection.However,Hallpike is to us, empiricallytrue.A relativelyslow group-selection
wrong if he thinks that only selection among self- process,graduallyculling througha myriadof complex
sufficientcommunitiescan lead to the spreadof group- alternativesocial arrangementsagainsta backgroundof
beneficialpractices. Mae Enga clans are embedded in ongoingtechnicalchange,can producesteadyimprovewhich,in turn,are embeddedin the Mae Enga mentsin cooperativecapabilitieswithoutreachingadapphratries,
ethnicgroup.At lower levels thereare subclans, fami- tive perfectionfor a very long time. Any mechanism
between which purportsto explain human social evolution over
lies, and so on. If thereare culturaldifferences
groupsat any level in this hierarchy,and ifthese differ- thepast 35,000 yearsor moremust account forall three
ences are heritableso thatdaughtergroupsare like their bodies of evidence. In generalit is a mistake to think
affectthe probabilityof thatadaptiveand historicalexplanationsare antithetical
parents,and ifthese differences
survivalor productionof daughtergroups,therewill be (Boydand Richersoni992). This mistakeleads to the
a selective process that leads to the spread of cultural absurddilemma that Hallpike erects.
Laland arguesthatculturalgroupselectionis unlikely
beliefsor practicesthat enhance the survivalor reprobehaviorbeduction of groupsat that level-that is, cultural attri- to lead to the evolution of self-sacrificial
butes thatbenefitgroupsnot individuals.The degreeto cause (i) most individually deleterious behaviors are
is relevantonly to the also deleteriousto the groupand (2) when behaviorsare
which groupsare self-sufficient
extentthatit affectsthe necessaryconditionsforgroup individuallydeleterioustheywill be rapidlyeliminated
survival or reproduction.Competition between firms by individual learningunless the conformisteffectis
can occur only in a particularculturaland institutional unrealisticallystrong.We disagree.There are many beenvironment,and as a resultbusiness firmsare not, in haviorsthatare individuallycostlybut group-beneficial.
They must recruitemployees, Such a payoffstructure(called an n-personprisoner's
any sense, self-sufficient.
importresources,and exportgoods or services.Nonethe- dilemma by game theorists,a public good by econoless, thereis much evidencethatpracticesthatenhance mists,and the tragedyofthe commonsby human ecolothe abilityof firmsto surviveand growspreadbecause gists)arises in a wide varietyof social contexts,includ-
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SOLTIS,
BOYD,
AND
RICHERSON
Group Selection: An Empirical Test I 49I
ing environmentalpollution,overexploitationof game selectionamong alternativestable equilibriaand found
resources,warfare,collec- thatit cannotgeneratethe variationnecessaryforgroup
and othercommon-property
tive political action, and many, many others.We also selection to work. The reason is that rare beneficial
believe that a weak conformisteffectcan stabilize indi- equilibria cannot reproducethemselves and therefore
processes.
viduallycostlybehaviorsagainst the effectsof individ- such traitsare eliminatedby within-group
Two commentatorsbelieve that the model of group
forindividualsto
ual calculationwheneverit is difficult
discernthe relativecosts of alternativebehaviors.Reli- selection we have outlined requiresthat groupsbe isogious beliefsprovidemany examples. Religious ideolo- lated from each other. Vayda argues that variation
gies create elaborately rationalized, difficult-to-doubtamonggroupscannot persistover long time periodsbein which heroes cause thereis frequentcontactand intermarriage
beliefssuch as thatthereis an afterlife
among
are rewardedand cowardsare punished.Dramatic cere- groupsand such contactwould rapidlydestroybetweenmonies and effectivesystemsofindoctrinationgenerate groupvariation.Knauft,in contrast,chides us forbeing
can pergreat emotional salience for such beliefs. As a conse- unnecessarilyconservative-group differences
quence, individual learningmay have little effecton sist, he argues,despite frequentcontact and intermartheimportheir spread. A group in which most people fervently riage-and as a consequenceunderestimating
in paradisemayhave tance of group selection. In fact, the model does not
believethatheroeswill live fci-ever
an advantage in conflictswith more rational groups assume thatfrequentcontactwill necessarilyerodevariation among groups.Rather,it assumes that this ten(RappaportI979).
Severalcommentatorssuggestthatour data are biased dency may be counteractedby the tendencyof people
in a way that underestimatesthe strengthof groupse- to adapt theirbeliefsand values to local circumstances.
lection. Peoples cautions that New Guinea may not be If these adaptive processes,which we have labeled birepresentative,Vayda suggests that many extinctions ased cultural transmissionand guided variation, are
may occur withoutwarfare,and Knauftnotes that the strong,theycan maintaindifferences
among groupsdelive in much moreflexible,fluidpopu- spite extensive mixing. Consider the followinghypohunter-gatherers
lation structuresthat may potentiategroup selection. theticalexample: There is a populationsubdividedinto
All ofthese objectionsare well taken.We have provided a numberofgroupswith frequentcontactand intermara single, rough upper-boundestimate on the rate of riage.In most groupsmost people believe thatnepotism
group selection for a particularformwhich seems to is morallycorrect.However, thereare a few groupsin
have characterizedNew Guinea societiesin thefirsthalf which most people believe that nepotismis wrongand
of this century.It would clearlybe verydesirableto de- should be punished. If adaptive processes are strong,
rive estimatesforpeople living in otherenvironments theycan maintainthissituationindefinitely.
New nepousing differentsubsistence techniques. Our estimate tisticimmigrantswho arrivein a groupin which nepomay be robust,or it may be that changesin the details tism is thoughtto be wrongrapidlyleam thatnepotism
ofhow theprocessworkswill turnout to be veryimpor- is a bad idea, so that when yet more nepotisticimmitant. As does Knauft,we think that estimatinggroup- grantsarrivethey findthemselvesto be in the minorselectionratesforhunter-gatherer
populationsis ofpar- ity and rapidlylearn to avoid nepotisticbehavior,thus
ticularimportancebecause it is plausible that cultural maintainingthe strategythatoriginallyarose merelyby
groupselection occurringover much of the Pleistocene chance.If,in contrast,adaptiveprocessesare weak, then
may have created social environmentswhich favored in a nonnepotisticgroup most of the immigrantswill
genetic altruism. When we began this project we at- believein nepotismand onlya fewimmigrantswill have
temptedto findcomparabledata forseveral otherareas rejectedtheirpreviousbeliefsby the time more immibut were unsuccessful.We hope that this paper may grantsarrive.This reduces the cost of behavingnepostimulateotherswho are more knowledgeablethan we tistically,and thus even fewerof these new immigrants
reject nepotism, nepotism eventually becoming the
are to publish additionalestimates.
Kawata suggeststhatintrademicgroupselectionmay norm and the subpopulation coming to resemble the
be fasterthanthe interdemicgroupselectionconsidered global population.
We believe that thereis ample evidencethat cultural
here. While we agree that theremay be otherformsof
groupselectionthatcould lead to morerapidsocial evo- differencesamong neighboringgroups can persist for
lution,we do not thinkit is likelythatintrademicselec- long periods of time. However, we also believe that it
tion could have this effect.Biologists distinguishbe- is importantto understandthe processes that maintain
tween interdemicgroupselection,in which long-lived, culturalvariationbecause theymayhave importantconpartlyisolated groups compete, and intrademicgroup sequences. In the present context,that extinctionreselection, in which competitionis among ephemeral sults fromthe disruptionof the group and that group
groupsthat are formedanew each generation.The cen- selectionrequiresthat new groupsbe formedby fission
tral problemwith both mechanisms is the generation both follow fromthe nature of the processes that are
ofvariationamonggroups.Many biologistsbelieve that assumed to maintaindifferences
amonggroups.If some
kinshipis the only mechanism that can generatesub- otherprocesses were importantin maintainingdifferstantialgeneticvariationamong the ephemeralgroups ences between groups,as is the case in most genetic
posited in intrademic models. Elsewhere, Boyd and models, then group selection would work quite differRicherson(unpublished)have modeledintrademicgroup ently.
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492
1 CURRENT
ANTHROPOLOGY
Volume 36, Number 3, June1995
That two specialists who have worked in the same AOKI, K. i982. A conditionforgroupselectionto prevailover
counteracting
individualselection.Evolution36:832-42.
regioncan disagreeso fundamentallyabout the nature
relationsin theFlyheadpoints up how BARTH, F. I97I. Tribesand intertribal
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