Nature, Nurture,
And Human
Diversity
PowerPoint®
Presentation
by Jim Foley
© 2013 Worth Publishers
Module 11: Behavior Genetics and
Evolutionary Psychology
Topics we were born to learn about
Behavior Genetics and Individual Differences
 Genes: Molecules that code for life
 Learning about heredity from Twin and
Adoption Studies
 Temperament and Heredity
 Molecular Genetics
Evolutionary Psychology: Adaptive Success
 Example: Explaining Mating Preferences
Behavior Genetics:
Predicting Individual Differences
The topics in the text:
1. genes
2. twin and adoption
studies
3. temperament and
heredity
4. molecular genetics
5. heritability
6. gene/environment
interaction
Behavior geneticists
study how heredity
and environment
contribute to human
differences.
Let’s start by looking
at GENES.
GENES:
The Building Blocks of
Heredity and
Development
Genes are
parts of DNA
molecules,
which are
found in
chromosomes
in the nuclei of
cells.
DNA
(Deoxyribonucleic Acid)
Chromosomes are made of DNA,
which are made of genes.
Chromosome:
threadlike structure made
largely of DNA molecules
DNA:
a spiraling, complex
molecule containing
genes
}
Chromosomes and Inheritance
 The human genome includes 46
chromosomes in 23 sets
matched sets; each
chromosome has the same
gene locations.
 This includes the X and Y
chromosomes, not a matched
set in males, who are missing
some genes on the Y.
 A biological parent donates half
his/her set of chromosomes to
his/her offspring.
 We received half a set of
chromosomes from each
biological parent.
The Human Genome:
20,000 to 25,000 Genes
 Human genomes are so nearly
identical that we can speak of one
universal human genome.
 Yet tiny genetic differences make a
difference. If there is a:
 .001 percent difference in genome,
your DNA would not match the
crime scene/you are not the
baby’s father.
 0.5 to 4 percent difference in
genome, you may be a
chimpanzee.
 50 percent difference in genome,
you may be a banana.
The genome: an
organism’s entire
collection of genes
How Genes Work
 Genes are not blueprints;
they are molecules.
 These molecules have the
ability to direct the
assembly of proteins that
build the body.
 This genetic protein
assembly can be turned on
and off by the
environment, or by other
genes.
 Any trait we see is a result
of the complex interactions
of many genes and
countless other molecules.
Next step for behavior geneticists:
Controlling Variables
Can we design an
experiment to keep genes
constant and vary the
environment and see what
happens?
Or vary the genes in the
same environment?
Fraternal and Identical Twins
Fraternal “twins” from separate eggs are not any
more genetically alike than other siblings.
Identical twin: Same sex only
Fraternal twin: Same or opposite sex
Twin and
Adoption Studies
To assess the
impact of nature
and nurture, how
do we examine
how genes make a
difference within
the same
environment?
 study traits of
siblings vs.
identical twins
 see if the
siblings vary
more than
twins
Identical vs. Fraternal Twins
Studies of twins in adulthood
show that identical twins are
more alike than fraternal twins in:
 personality traits such as
extraversion (sociability) and
neuroticism (emotional
instability).
 behaviors/outcomes such as
the rate of divorce.
 abilities such as overall
Intelligence test scores.
Twin and
Adoption Studies
How do we find
out how the same
genes express
themselves in
different
environments?
We can study the
traits of identical
twins as they grow
up, or if they were
raised separately
(e.g., the
Minnesota Twin
Family Study).
Studies of Identical Twins Raised Apart
Critiques of Twin Studies
Similarities found in
identical twins despite 1. In the more recent years of the
Minnesota Twin Family Study, twins
being raised in
have known about each other and
different homes:
may influence each other to be more
similar.
 personality, styles of
happen; some
thinking and relating 2. Coincidences
randomly chosen pairs of people will
have similar traits, including even
 abilities/intelligence
spouses, children, and dogs with
test scores
identical names.
 attitudes
3. Environments may be similar;
adoptive families tend to be more
 interests, tastes
similar than randomly selected
families in education, income, and
 specific fears
values.
 brain waves, heart
BUT none of these factors explains,
better than the genetic explanation,
rate
why fraternal twins have more
differences than identical twins.
Searching for Parenting Effects:
Biological vs. Adoptive Relatives
Studies have been performed with adopted children for
whom the biological relatives are known.
Findings: Adopted children seem to be more similar to
their genetic relatives than their environmental/nurture
relatives.
Given the evidence of genetic impact
on how a person turns out,
does parenting/nurture
make any difference?
Does the home environment have
any impact?
Parenting Does Matter
Despite the strong impact of
genetics on personality, parenting
has an influence on:
 religious beliefs
 values
 manners
 attitudes
 politics
 habits
If parenting has an influence,
why are siblings so different?
 Siblings only share half
their genes.
 Genetic differences
become amplified as
people react to them
differently.
 Siblings are raised in
slightly different families;
the youngest has more
older siblings and has
older (wiser? more
tired?) parents.
Temperament is another difference
not caused by parenting.
 From infancy into adulthood,
most people do not seem to
change temperament (defined
as a person’s general level and
style of emotional reactivity).
 According to some researchers,
three general types of
temperament appear in infancy:
• “easy”
• “difficult”
• “slow to warm up”
Molecular Genetics
 Molecular genetics is the study of the molecular structure and
function of genes.
 Molecular genetics might help us see exactly how specific genes
have an influence on behavior.
 Genetic tests can reveal which people are at risk for many
physical diseases, and may soon identify people at risk of
mental health disorders.
 Ethical conundrum: should people use genetic tests to select
sperm, eggs, and even embryos?
Heritability
 When you see a variation of
some trait within a
population, the heritability
of that trait is the amount of
variation in the population
that is explained by genetic
factors.
 This DOES NOT tell us the
proportion that genes
contribute to the trait for any
one person.
 The heritability of a trait also
does not tell us whether
genetics explain differences
between
groups/populations.
Clarifying Heritability
 If five unrelated people
had nearly identical
upbringing, but differed
in a trait such as shyness,
then the heritability of
this trait for them is
close to 100 percent.
 Nurture may have
influenced how shy they
are, but because it
influenced them all in
the same way, any
differences are almost
certainly caused by
genes.
Nature and nurture working together
Interaction of Genes and Environment
 Some traits, such as the overall design of our bodies, are set by
genes.
 Other traits, such as physical and mental abilities, develop in
response to experience.
How does the interaction of genes and environment work?
 Genetic traits influence the social environment, which in turn
affects behavior.
How does the interaction of genes and
environment work?
 Self-regulation: genes
turn each other on and
off in response to
environmental
conditions
 Epigenetics: the
environment acts on the
surface of genes to alter
their activity
Example of selfregulation in
animals:
shortened daylight
triggers animals to
change fur color or
to hibernate
Example of selfregulation in
humans:
obesity in adults
can turn off
weight regulation
genes in offspring
The Human Approach
to Nature and Nurture
 The trait of being adaptable is built into the human
genome.
Paradox: our genes allow us not to be tied so much
to our genes!
 We have minds which allow us to change our behavior
in response to the environment to a greater degree
than other species.
 We even shape our environments to suit our nature.
 Humans can adapt to a variety of climates, diets,
lifestyles, and skills.
Evolutionary Psychology:
Understanding Human Nature
Some topics:
 Natural selection and
adaptation
 Evolutionary success may help
explain similarities
 An evolutionary explanation
of human sexuality
Evolutionary
psychology is the
study of how
evolutionary
principles help explain
the origin and
function of the human
mind, traits, and
behaviors.
We have been talking so far about
human differences; we may now
seek insight in the ways in which
humans are alike.
23
Evolutionary Psychology:
Natural Selection: How it Works
Begin with a
species’ genome,
which contains a
variety of versions
of genes that
shape traits.
Conditions make it
difficult for
individuals with
some traits (some
versions of those
genes) to survive
long enough to
reproduce.
Other individuals
thus have their
traits and genes
“selected” to
spread in the
population.
Artificial Selection
The Domesticated Silver Foxes
 Dmitri Balyaev and
Lyudmila Trut spent
40 years selecting
the most gentle,
friendly, and tame
foxes from a fox
population, and
having those
reproduce.
 As a result, they
were able to shape
avoidant and
aggressive creatures
into social ones, just
as wolves were once
shaped into dogs.
25
How might evolution have shaped
the human species?
Example:
Why does “stranger anxiety”
develop between the ages of 9
and 13 months?
Hint: in evolutionary/survival
terms, humans are learning to
walk at that time.
Possible explanation: infants
who used their new ability to
walk by walking away from
family and toward a lion might
not have survived to reproduce
as well as those who decided to
cling to parents around the
time they learned to walk.
26
Evolutionary Psychology’s
Explanation of Phobias
 Why do people so easily
acquire a phobia of
snakes?
 An evolutionary
psychologist would note
that snakes are often
poisonous…
so those who more readily
learned to fear them were
more likely to survive and
reproduce.
 Can we apply the same
logic to phobias about
heights? enclosed
spaces? clowns?
27
A Possible Human Genetic Legacy:
“Illogical” Moral Reasoning
It might be “logical” to kill one innocent person if it
would enable five other innocent people to live.
Research shows that most people can imagine letting
the one person die, but cannot picture killing the
person themselves.
Why would it be
instinctual not to kill
unless we are directly
threatened?
Male and Female Differences:
Focusing on Mating Preferences
First issue: quantity (of mating)
 Generally, men think more than women about sex, and men
are more likely to think that casual sex is acceptable.
 Why might natural selection have resulted in greater male
promiscuity?
An evolutionary psychologist’s answer:
Men who had the trait of
promiscuity were more likely
to have their genes continue,
and even spread, in the next
generation. And there is little
cost to spreading extra
genes.
For women, a trait of
promiscuity would not
greatly increase the number
of babies, and it would have
greater survival costs
(pregnancy, once a lifethreatening condition).
Possible Evolutionary Strategies in
Seeking Partners
Q: How would evolutionary psychology
explain why males and females have
different preferences for sexual partners?
Men seek women with a
fuller figure…
to make sure they are
not too young or too old
to have children?
Women seek males with
loyal behavior and
physical/social power and
resources…
in order to ensure the
survival of the mother’s
offspring?
Critiquing the Evolutionary Perspective on
Gender Differences in Sexuality
Are males and
female really so
different in their
mating choices?
Differences are less
in cultures that move
to gender equality.
Isn’t much of
gender behavior a
function of culture?
Yes, as we’ll see later
in this lesson.
How do you
explain
homosexuality?
Guesses such as
population control or
misplaced instincts
are unproven and
seem forced.
Does evolutionary
psychology really
tell us anything
useful?
See next
slide…
Critiquing Evolutionary Psychology
“You’re just
taking current
reality and
constructing a
way you could
have predicted
it.” This is
hindsight
reasoning and
unscientific.
Response: yes, but
there are predictions
made about future
behavior using this
reasoning.
“You’re
attributing too
much to genes
rather than the
human ability to
make choices
about social
behavior.”
Response: yes, but our
evolutionary past does
not prevent our ability to
act differently; “is” does
not equal “ought.”