Chapter 4
Nature,
Nurture, and
Human
Diversity
Overview:
Nature/Nurture/Diversity Questions
 How do we explain traits that all humans may
have in common?
 How do we explain the origins of diversity,
the source of differences in the traits:
• between genders?
• among cultures?
• among individuals?
 But first, how do we investigate these issues?
• Genetics
– Branch of biology that deals with the
mechanisms of heredity.
• Heredity
– Transmission of genetic information from
one generation to the next
• Behavior Genetics
– Study of how variation in genes affects
variation in behavior
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
}
DNA
• Watson and Crick (1953)
• Identified the molecule’s structure
– Double helix
– 4-letter alphabet code: A,G,T,C= 4 Nucleotides
– Mechanism of replication discovered
– DNA = an amazing little replicator
• Nobel Prize (1962)
"for their discoveries concerning the molecular
structure of nucleic acids and its significance for
information transfer in living material"
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.
24
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.
26
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.
27
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?
28
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 life-threatening 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.”
Nature/Nurture
From Genes to the Role of Environment
 How environment/experience affects brain development
 Forces guiding the course of development:
 parents
 peers
 culture
Our starting picture
Our environment gives us
our experiences.
Experience and Brain Development
Rats living in an “enriched” environment (more
social interaction and physical play) experienced a
greater growth in brain size and complexity than
those rats living in an “impoverished” environment.
Brain Development Means
Growth AND Pruning
 To make our well-used brain
pathways work better, the
unused connections are
“pruned” away.
 This means that if certain
abilities are not used, they will
fade.
Impact of Experience/Nurture on
Brain Development
The Process Continues into Adulthood
Repeated practice at
a finger-tapping task
begins to activate a
[slightly] larger group
of motor neurons.
Is parenting a powerful
environmental influence on
development?
 Generally,
environmental
influences, including
parenting, account for
about 10 percent of
temperament,
although a much
higher percentage for
other features such as
values.
 Non-abusive “average”
parents should ease
off on both the blame
and the credit they
assume for how their
kids turn out.
Where this percentage increases:
“extreme” parenting, including
severe neglect and abuse
Peer Influence
 The degree of peer influence is hard to trace. Apparent
conformity (the whole group smokes) could be a selection effect
(they get together because they want to be with others who like
to smoke).
 Interaction with peers can teach new social skills.
 Parents may try to have indirect influence by selecting a child’s
peers, such as by selecting a school or neighborhood. However,
ultimately, most children self-select their peers.
Parents vs. Peers
Battling over non-genetic influence
Parents have more
influence on:
Education and career path
Cooperation
Self-discipline
Responsibility
Charitableness
Religion
Style of interaction with
authority figures
Peers have more
influence on:
Learning cooperation skills
Learning the path to popularity
Choice of music and other
recreation
Choice of clothing and other
cultural choices
Good and bad habits
Culture Influences on Development
The
nature of
culture
Variation
across
cultures
Examples
of
cultural
variation
over time
 Culture refers to the patterns of ideas, attitudes, values,
lifestyle habits, and traditions shared by a group of
people and passed on to future generations.
 Culture is not just an influence on our nature, but it is
also part of our nature. Humans form not only
relationships, but culture.
 Each culture has norms--standards for acceptable,
expected behavior.
 Example: “Eww, you wear your shoes from outdoors right
into the house?”
 Culture shock: feeling lost about what behaviors are
appropriate
Cultural variation can occur even within one culture:
 language changes in vocabulary and pronunciation
 the pace of life quickens
 gender equality increases.
 people sleep less, socialize in person less, stare at screens
more
 people marry more for love, but then expect more
romance
These cultural changes occur too fast to be rooted in genetic
change.
Culture Influences on Development
Culture and the self: individualism and collectivism
 Individualist cultures value independence. They promote personal
ideals, strengths, and goals, pursued in competition with others,
leading to individual achievement and finding a unique identity.
 Collectivist cultures value interdependence. They promote group
and societal goals and duties, and blending in with group identity,
with achievement attributed to mutual support.
Individualist and Collectivist Cultures Compared
Culture Influences on Development
Similarities across groups
 Although there are cultural differences, the differences within any
group are usually greater than the differences between groups.
Example: How socially active are people in people in two
hypothetical countries? They may differ on average because of
cultural influence, but both countries may have many mildly friendly
people.
Sociability Levels in Shyland
Sociability Levels in Partyland
SCALE: Self-Isolating -- Shy --
Private -- Friendly -- Outgoing -- Partier
Level/Amount of Social Activity
Culture and Genes:
A Complex Interaction
 There is a difference in average
blood pressure between
“racial” groups.
 This may seem like a genetic
difference but may actually be
a cultural difference…. How?
Different cultures may
have dietary
differences, which in
turn affect blood
pressure.
Child-rearing: Cultural Differences
 People in individualist
cultures might raise
children to be self-reliant
and independent.
 People in collectivist
cultures might raise
children to be compliant,
obedient, and integrated
into webs of mutual
support.
 People in Asian and
African cultures might
raise children to be more
emotionally and
physically close to others
than in western
European cultures.
Gender Development
Gender refers to the physical, social, and
behavioral characteristics that are culturally
associated with male and female roles and
identity.
Some of these traits may be genetic differences;
other role differences may be nurtured by culture.
Group differences?
In this example related to self-esteem, the difference
between groups is small compared to differences
within each gender.
Differences Between Genders
Biological:
women enter
puberty
earlier, live
longer, and
have more fat
and less
muscle
Mental and Behavioral Health:
 women are more likely to have
depression, anxiety, or eating
disorders
 men are more likely to have
autism, ADHD, and antisocial
personality disorder
Gender and
Aggression:
 men behave more
aggressively than
women, and are
more likely to
behave in ways that
harm others
 this difference
applies to physical
aggression rather
than verbal or
relational aggression
Gender and Social Power
 In a variety of cultures,
men have attributes
and reputations that
help them attain more
social power (positions
controlling more people
and resources) than
women do.
 Men tend to interact in
more dominating ways
than women. Men
often speak opinions
rather than offering
support and inviting
input as women do.
Gender and Social Connection: Play
 When boys play, the focus
tends to be on the activity.
 Male play is more competitive.
 Men tend to dictate how the
playtime will proceed.
 When women play, the focus
tends to be on connection and
conversation.
 Female play is more social.
 Girls tend to invite feedback.
Are these differences due to nature or nurture?
Gender and Social Communication
Women communicate
more than men:
 more time with friends
 more text messages
 longer phone calls
However, men and
women speak about the
same number of words
per day. What fills in the
extra time on those
longer phone calls?
Maybe….
listening?
Men and women use communication differently.
 Women seek input and
 Men state their opinions
explore relationships.
and solutions .
 Men speak about things and  Women speak about people
and feelings.
actions.
Gender and Social Connectedness
 Both men and women
turn to women when
they want someone
to talk to, seeking the
“tend and befriend”
response or better
listening.
 In general, women
change roommates
more often.
 Women tend to have
stronger ties to
friends and family.
 Women are often
more involved with
religion.
The Biology of Gender
What biologically makes us
male or female?
 It begins with whether our
23rd pair of chromosomes
looks like XX (female) or
Xy (male).
 Testes develop, and at
seven weeks, the testes
produce a flood of
testosterone.
 Hormones then guide the
development of external
sex organs.
Brain Differences
 During the fourth and
fifth month of
pregnancy, sex
hormones bathe the
fetal brain.
 In adulthood, women
have thicker areas in a
part of the frontal lobes
that help with verbal
fluency.
 There are also
differences in the
amygdala,
hippocampus, and ratio
of cell bodies to axons.
Lessons about Gender:
Unusual Biological Cases
 In cases in which prenatal
testosterone levels were
high in females, there is
an increase in
“tomboyish” behavior,
possibly caused by other
people’s response to
more masculine features.
However, there is not a
general pattern of gender
identity change.
 In cases where males had
underformed or absent
genitalia, attempts to
raise them as females
generally did not work
out well.
Breaking Free of Gender
 Transgendered people have a
sense of sexual identity that is
different from their birth sex.
 Transsexual people act on this
sense of difference by living as a
member of the opposite sex,
often with hormonal and surgical
interventions that support this
gender reassignment.
The Nurture side of Gender Roles:
The Influence of Culture
Gender role: the behaviors
expected of people related to
their identity as men and
women
Gender identity: one’s sense of
whether one is male and female,
including a sense of what it means
to be that gender
Does culture define which behaviors fill a gender role?
Or do the roles affect culture?
Gender roles and culture: is differentiation a good thing?
If it’s man’s job to get
the high paying
employment,
If it’s women’s work
to take care of the
kids and home,
 does that prevent conflict, and help culture stay stable,
because roles are clear?
 or is equality worth having some conflict and uncertainty?
Change in Social Roles?
If current trends continue, women will soon be the
majority of practitioners in some fields that were once
dominated by men in the United States.
Culture Influence
on Gender Role Development
Or is it instinct?
 Social learning theory: we learn gender role
behavior by imitation, and by rewards and
punishments that shape our behavior
 Gender schemas: the cognitive frameworks for
developing concepts of “male” and “female”;
these frameworks guide our observations
 Gender typing: the instinct which drives some
children to fit into traditional gender roles
Influences on Who You’ve Become
Beyond Biopsychosocial Influences:
CHOICE
 Is our behavior and identity rigidly
determined by our combination of
nature/genes and nurture/experience?
Even if free will is an illusion, it would seem
that we can make choices that override our
genetic influences, that differ from cultural
norms, and that do not fit our environment.
We can even choose to directly alter
culture, environment, and even genes.
Epilogue: Evolution
 Evolution is a scientific theory
(NOT a guess, and not a
hypothesis, but something more):
a coherent set of principles that
fits very well with the accumulated
evidence.
 Parts of the evolutionary story
may conflict with other stories of
origins and change over time.
 Is there room for overlap and
agreement?
Possible areas of
consensus, with or
without evolution:
 The human mind
and body seems
almost
“designed,” by
evolution or other
forces, to have
certain traits and
abilities.
 Nurture may
shape us, but we
seem to start out
with some sort of
human nature.