Chapter 4
Nature,
Nurture, and
Human
Diversity
PowerPoint®
Presentation
by Jim Foley
© 2013 Worth Publishers
Behavior Genetics:
Predicting Individual Differences
The topics in the text:
1.genes
2.twin and adoption
studies
3.temperament and
heredity
4.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.
Chromosome: threadlike
structure made largely of
DNA molecules
DNA: a spiraling, complex
molecule containing genes
DNA
(Deoxyribonucleic Acid)
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.
How Genes Work
 Genes are not blueprints;
they are molecules that
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,
gene expression, or by other
genes.
 Any trait we see is a result of
the complex interactions of
many genes and countless
other molecules.
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
Similarities found in
identical twins despite
being raised in
different homes:
personality, styles of
thinking and relating
abilities/intelligence
test scores
attitudes
interests, tastes
specific fears
brain waves, heart
rate
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?
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.
Searching for Parenting Effects:
Biological vs. Adoptive Relatives
Despite the strong impact of
genetics on personality,
parenting has an influence on:
 religious beliefs
 values
 manners
 attitudes
 politics
 habits
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”
Nature and nurture working together
Interaction of Genes and Environment
 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
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.
12
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.
14
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.
15
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?
16
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?
Nature/Nurture
From Genes to the Role of Environment
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.
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
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
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.
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 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.
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.