CITS4403Computa/onal
Modelling
AgentBasedModels
Agents
Agentsrepresenten//esinasystemwhichareabletoperform
ac/ons,andrespondtolocalinforma/onaccordingtoasetof
rules.
IntelligentsoCwareagentsareoCendesignedaroundtheBeliefDesire-Inten-onmodel.
Mul/-agentsystemsareabletoachievecomplexgoalsusing
rela/velysimpleagents.
Agentbasedmodelsareaneasywaytomodeltheeffectsof
basicassump/ons.
Segrega/on
TheSchellingmodeloftheworldisagrid;eachcellrepresentsa
house.Thehousesareoccupiedbytwokindsof“agents,”
labeledredandblue,inroughlyequalnumbers.
Atanypointin/me,anagentmightbehappyorunhappy,
dependingontheotheragentsintheneighborhood.The
neighborhoodofeachhouseisthesetofeightadjacentcells,
andagentsareunhappyiftheyhavelessthan2neighbourslike
themselves.Ifthey’reunhappy,theymovetoanarbitraryempty
cell.
Emergentbehaviour
Fromverysimplerulesweno/cesegrega/onasanemergent
property.
Noteagentsarenotopposedtotheothertype,theyarejust
seekingfamiliarity.
Whatisagoodmeasureofsegrega/on?
Agent-BasedProgramming
AgentOrientedProgrammingisanprogrammingparadigmthat
usessoCwareagentsworkingtowardslocalgoalstoachieve
globalresults.
Netlogo(hWp://ccl.northwestern.edu/netlogo/)isagood
exampleofanagentorientedlanguage,whichhasanexcellent
modellibrary:
Agent-basedmodels
•  Thecharacteris/csofagent-basedmodelsinclude:
•  Agentsthatmodelintelligentbehavior,usuallywithasimple
setofrules.
•  Theagentsareusuallysituatedinspace(orinanetwork),and
interactwitheachotherlocally.
•  Theyusuallyhaveimperfect,localinforma/on.
•  OCenthereisvariabilitybetweenagents.
•  OCentherearerandomelements,eitheramongtheagentsor
intheworld.
TrafficJams
Trafficmodelsareaclassicexampleofagentbasedmodelling.
•  TheHighwayisaone-laneroadthatformsacircle,butitis
displayedasaseriesofrowsthatspiraldownthecanvas.
•  Eachdriverstartswitharandomposi/onandspeed.Ateach
/mestep,eachDriveracceleratesorbrakesbasedonthe
distancebetweenitandtheDriverinfront.
Boids
Agentsinthismodelsarecalled“boids”.Eachagentsimulates
threebehaviors:
•  Collisionavoidance:avoidobstacles,includingotherbirds.
•  Flockcentering:movetowardthecenteroftheflock.
•  Velocitymatching:alignvelocitywithneighboringbirds.
Boidsmakedecisionsbasedonlocalinforma/ononly;eachboid
onlysees(orpaysaWen/onto)otherboidsinitsfieldofvision
andrange.
Prisoner’sDilema
Twosuspects[AliceandBob]arearrestedbythepolice.The
policehaveinsufficientevidenceforaconvic/on,and,having
separatedtheprisoners,visiteachofthemtoofferthesame
deal.Ifonetes/fiesagainsttheother(defects)andtheother
remainssilent(cooperates),thedefectorgoesfreeandthesilent
accomplicereceivesthefullone-yearsentence.Ifbothremain
silent,bothprisonersaresentencedtoonlyonemonthinjailfor
aminorcharge.Ifeachbetraystheother,eachreceivesathreemonthsentence.
IteratedPrisonersDilema
Whatisthebeststrategyistheprisonersdilema?
Whatifyouhadtorepeatedlyplaytheprisoner’sdilemawith
thesameperson,orwiththesamepopula/on?
Inthelate1970sRobertAxelrodorganizedatournamentto
comparestrategies.Par/cipantstosubmiWedstrategies,then
playedtheprogramsagainsteachotherandkeptscore.
Emergence
Anemergentpropertyisacharacteris/cofasystemthatresults
fromtheinterac/onofitscomponents,notfromtheir
proper/es.
Conven/onalscienceisbasedontheaxiomthatifyouknowthe
rulesthatgovernasystem,youcanpredictitsbehavior.
Howeversomesystemsarecomputa-onallyirreducible:theonly
waytopredicttheirproper/esistoexecutethesystem.
Tosomepeople,“emergence”isanothernameforignorance:a
propertyisemergentifwedon’thaveareduc/onistexplana/on
forit.
EmergentProper/es
Manycomplexsystemshaveproper/es,asawhole,thattheir
componentsdonot:
•  TheRule30cellularautomatonisdeterminis/c,andtherulesthat
governitsevolu/onarecompletelyknown.Nevertheless,it
generatesasequencethatissta/s/callyrandom.
•  TheagentsinSchelling’smodelarenotracist,buttheoutcomeof
theirinterac/onsisasiftheywere.
•  Trafficjamsmovebackwardeventhoughthecarsinthemare
movingforward.
•  Thebehaviorofflocksandherdsemergesfromlocalinterac/ons
betweentheirmembers.
•  AsAxelrodsaysabouttheiteratedprisoner’sdilemma:“The
emergenceofcoopera/oncanbeexplainedasaconsequenceof
individual[s]pursuingtheirowninterests.”
FreeWill
Freewillistheabilitytomakechoices,butif
ourbodiesandbrainsaregovernedby
determinis/cphysicallaws,ourac/ons
wouldbedetermined.Someargumentsare:
•  WilliamJamesproposedatwo-stagemodelinwhichpossible
ac/onsaregeneratedbyarandomprocessandthenselected
byadeterminis/cprocess
•  DavidHumesuggestedthatourpercep/onofmakingchoices
isanillusion;inthatcase,ourac/onsaredeterminis/c
becausethesystemthatproducesthemisdeterminis/c.
•  RogerPenroseproposesthatfreewillcouldbeaconsequence
ofquantumuncertainty.