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Nature, Nurture, and Human Diversity
Differences and similarities within the human family
Similarities
Differences
Genes: Same set of
chromosomes
Genes: Genetic anomalies
may make us different
Biology: Organs and body
functions same
Biology: May change during
development
Brain: Same brain architecture
Brain: Asymmetry of brain
across genders
Behaviors: Speak language
Behavior: Speak different
languages
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.
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.
Dominant and Recessive
Genotype—underlying genetic makeup
 Phenotype—traits that are expressed
 Dominant genes—will always be
expressed if present
 Recessive genes—will not be expressed
unless they are in a pair

Sex Linked Traits
Traits linked to the X or Y (sex)
chromosomes
 Usually recessive and carried on the X
chromosome
 Appear more frequently in one sex than
another
 Color blindness, baldness, hemophilia,
Fragile X

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
Critiques of Twin Studies
1. In the more recent years of the
Minnesota Twin Family Study, twins
have known about each other and
may influence each other to be
more similar.
2. Coincidences happen; some
randomly chosen pairs of people will
have similar traits.
3. Environments may be similar;
adoptive families tend to be more
similar than randomly selected
families in education, income, and
values.
BUT none of these factors explains,
better than the genetic explanation,
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
How does the interaction of genes
and environment work?

Gene-Environment
Interaction: genes turn
each other on and off in
response to
environmental
conditions
Example in
animals:
shortened daylight
triggers animals to
change fur color or
to hibernate

Epigenetics: The study
of how this happens:
The environment acts
on the surface of genes
to alter their activity
Example in
humans:
obesity in adults
can turn off weight
regulation genes in
offspring
Behavior Genetics

Temperament
a person’s characteristic emotional
reactivity and intensity
 Identical twins express similar
temperaments, suggesting
heredity predisposes temperament

Behavior Genetics

Heritability

the proportion of variation among
individuals that we can attribute to
genes
 Heritability refers to the extent to
which the differences among
people are attributable to genes.
Group Differences

Group differences and environmental impact
Variation within group
Variation within group
Seeds
Poor soil
Fertile soil
Difference within group
Group Differences
If genetic influences help explain individual
diversity in traits, can the same be said about
group differences?
Not necessarily. Individual differences in weight
and height are heritable and yet nutritional
influences have made westerners heavier and
taller than their ancestors were a century ago.
Prenatal Environment
Identical twins who share the same placenta (b)
are more alike than those who do not (a),
suggesting prenatal influences on psychological
traits.
Experience and Brain Development
Early postnatal experiences affect brain
development. Rosenzweig et al. (1984) showed
that rats raised in enriched environments
developed thicker cortices than those in
impoverished environment.
Brain Development and Adulthood
Brain development does not stop when we
reach adulthood. Throughout our life, brain
tissue continues to grow and change.
A well-learned finger-tapping task leads to
more motor cortical neurons (right) than baseline.
Variation Across Culture
Cultures differ. Each culture develops norms –
rules for accepted and expected behavior. Men
holding hands in Saudi Arabia is the norm (closer
personal space), but not in American culture.
Jason Reed/ Reuters/Corbis
Environmental Influence

Personal Space


the buffer zone we like to maintain
around our bodies
Memes

self-replicating ideas, fashions, and
innovations passed from person to
person
Variation Over Time
Cultures change over time. The rate of this
change may be extremely fast. In many
Western countries, culture has rapidly changed
over the past 40 years or so.
This change cannot be attributed to changes in
the human gene pool because genes evolve
very slowly.
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.
Evolution: Theory



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.
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.
Gender differences and
similarities
Compared to men, women (as a
group, on average):
• Start puberty 2 years sooner
• Live 5 years longer
• Have 70% more fat
• Have 40% less muscle
• Are 5 inches shorter
• Express more emotion
• Are more vulnerable to
anxiety, depression
• Are less prone to autism,
alcohol dependence, ADHD,
antisocial personality, and
suicide
Similarities:
45 out of 46 genes
are unisex
Same body and
brain structures
and functions
Similar levels of
Intelligence,
knowledge,
happiness
Average/Group differences do
not predict individual
comparisons
In this example related to self-esteem, the
difference between groups is small compared to
differences within each gender.
This means: many
women rate higher
than the average
man, in self esteem
and other
measures.
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.
And yet violent acts
by women do occur,
including acts of
terrorism.
Bombing in Russia by
female suicide bombers
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
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
explore relationships.
opinions and solutions .
 Men speak about things  Women speak about
people and feelings.
and 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.
Biology of Sex
Biological sex is determined by the twenty-third pair
of chromosomes. If the pair is XX, a female is
produced. If the pair is XY, a male child is produced.
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.
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?
do the
roles affect
culture?
Gender roles and Or
culture:
Expectations
may
vary
In North American
societies, men have
been providers, women
were caretakers
In some societies, men
and women share more
in child rearing and
accumulating resources
 Gender roles have simplified, yet constrained, choices for
men and women.
 In the past century, women have been gaining more options
for participation in workplaces and politics.
Culture Influence
on Gender Role Development



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
Gender Schema Theory
Gender-role development is influenced by the
formation of schemas, or mental
representations, of masculinity and femininity
An example of how a child forms a schema
associated with gender. A girl is offered a choice of 4
toys to play with.
Boys
Toy car
Approach
object
Doll
Who
for?
Not
for me Avoid/
Is it relevant
Forget
to me?
I am a girl
Orange
Artichoke
Girls
Assign to
category
For me and
remember/
Approach
The Nature and Nurture of Gender

Two theories of gender typing
Variations in Sexual Identity
Breaking free of gender roles
 Transgendered people have a sense of sexual
identity (sense of being male or female) or gender
expression (behaviors and appearance that
express gender identity) that is different from what
is culturally typical for the biological sex/gender
they were born with.
 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.
Hormones and Sexual Behavior





Hormones such as estrogens
(female sex hormones) and also
in testosterone (male sex
hormones) guide the physical
development of sex
characteristics and behaviors.
During ovulation, women show
a rise in both sex hormones.
As this happens, sexual desire
rises in women and also in the
men around them (whose
testosterone level rises).
Low levels of testosterone can
reduce sexual motivation.
Sex hormone levels fall with
age (menopause), drugs, or
surgery.
Intelligence
Males and females are very close in overall
intellectual abilities.
Spatial Skills
Skills involving the ability to
imagine how objects would look
when moved about in space.
Males generally do better though
social role, social class, ethnic
background, and the type of test
given also play roles.
Social Factors
Self-Confidence
In adolescence, females’ confidence often declines
(concerned with how boys will react to them?).
The loss of confidence is highest in whites and
Hispanics and lowest in African Americans.
Overall, studies have not found major differences
in self-confidence levels between males and
females.
Friendships
 Boys’ groups usually have a leader, girls
have more equal status in groups.
 Boys usually issue orders and demands
while girls make more suggestions.
 Adult females tend to have a best friend of
the same sex while males do not.
 Adult females have a few close friends while
males have a larger number of male
acquaintances.
 Females initiate most breakups.
Selecting a Mate
1. Social Status
Overall, the top eight
characteristics for
potential mate selection
are:
2. Economic Status
3. Physical Attractiveness
4. Personality
5. Intelligence
6. Ambition
7. Character
8. Sense of Humor
Gender Differences in Sexuality:
An Evolutionary Perspective
• 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).
Natural Selection and
Mating Preferences
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?
Gender Role Behavior
Gender role behaviors reflect what society says
are appropriate actions for males and females.
Through identification with a parent of the same
sex, people learn how to behave appropriately.
Which jobs are
appropriate
for males?
for females?
Gender studies have shown
Fathers who are very warm
toward daughters tend to produce
mild tomboyishness in girls.
Boys whose fathers were absent
during preschool years tend to be
less aggressive and not as active
in sports.
People who are considered mentally healthy tend not to
have excessive masculine or feminine qualities.
Children tend to identify with the dominant parent in
the household, even across sex lines.
Fathers are more determined that appropriate
activities are given to young male children.
People handle babies differently depending on
whether they are identified as male or female.
People will assign different characteristics
to babies when they are identified as male
or female.
Mixing Gender Roles
Rigid extremes for gender roles for males and
females restrict the full range of human
behaviors and emotions.
Androgynous people are high in male and
female characteristics.
Androgynous behavior can lead to more
flexibility and willingness to share
characteristics of members of
the opposite sex.
Gender-Busting Communication
Hints (for Males)
1. Do you frequently interrupt females while they are
speaking? Try to break the habit.
2. Avoid answering questions with “Nope”and “Yep.” Try
to explain more and give some details why you did
something.
3. Try to open up on personal issues.
4. Ask for help if you need it.
Can you tell me
how to get to . . .?
Gender-Busting Communication
Hints (for Females)
1. Look people directly in the eye.
2. If men interrupt conversations, assertively direct the
conversation back.
3. Do not over-apologize for your behaviors.
4. Talk more often about current events,
less about people.
Reflections on Nature and Nurture