module 11
Behavior Genetics and
Evolutionary
Psychology
Module Preview
Members of the human family share common behavioral
tendencies but are also strikingly diverse. To what
extent are we shaped by our heredity and to what degree
by our life history? The conclusions—that nature is
crucially important and that nurture is crucially
important—are central to today’s psychology.
Genes provide the blueprints that design both our
universal human attributes and our individual traits.
Behavior geneticists explore individual differences. By
using twin, adoption, and temperament studies, they
assess the heritability of various traits and
disorders. Their research indicates that both nature
and nurture influence our life courses. We are products
of interactions between our genetic predispositions and
our surrounding environments. Molecular geneticists
search for genes that put people at risk for
genetically influenced disorders, which has potential
benefits as well as risks.
Evolutionary psychologists focus on what makes us
alike as humans. They study how natural selection
favored behavioral tendencies that contributed to the
survival and spread of our genes. For example, in
explaining gender differences in sexual behavior, they
argue that women most often send their genes into the
future by pairing wisely, men by pairing widely.
Critics maintain that evolutionary psychologists make
too many hindsight explanations and underestimate the
role of culture.
Module Guide
Introductory Exercise: Fact or Falsehood?
Lecture: Universal People
11-1. Give examples of differences and similarities within
the human family, noting how they are shaped by our
genes and our environment.
Members of the human family differ in personality,
interests, culture, and family background. At the same
time, our shared brain architecture predisposes us to
sense the world, develop language, and experience
hunger through the same mechanisms. Humans everywhere
affiliate, conform, reciprocate favors, punish
offenses, and grieve a child’s death. Our genes and our
environment contribute to our striking diversity and
shared human nature.
Behavior Genetics: Predicting Individual Differences
Lectures: The Origins of Blue Eyes; The Genetic Revolution
Exercise: Genetic Factors
Exercise/Project: Genetic Influences
Instructor Video Tool Kit: Ethics in Human Research: Violating
One’s Privacy
11-2. Identify the types of questions that interest behavior
geneticists, and describe the elements of heredity:
chromosome, DNA, genes, and genome.
Behavior geneticists study our differences and aim to
determine the relative importance of heredity and
environment on behavior. Environment includes every
nongenetic influence, from prenatal nutrition to the
people and things around us.
Every cell nucleus contains the genetic master code for
the body. Within each cell are 46 chromosomes with 23
donated by each parent. Each chromosome is composed of
a coiled chain of a molecule, called DNA
(deoxyribonucleic acid). Genes are DNA segments that,
when “turned on” (active or expressed), provide the
code for the production of protein molecules. By
directing the manufacture of proteins, the
approximately 30,000 genes that compose the human body
determine our physical development. The genome provides
the complete instructions for making an organism,
consisting of all the genetic material in the
organism’s chromosomes. Variations at particular gene
sites in the DNA define each person’s uniqueness. Human
traits are influenced by many genes interacting with
the environment.
Lectures: The Minnesota Twin Study; “Mom Always Liked You Best”
Exercise: Striking Similarities
Videos: Module 16 of The Mind series, 2nd ed.: The Effect of Aging
on Cognitive Functioning: Nature/Nurture
Instructor Video Tool Kit: Nature Versus Nurture: Growing up Apart
11-3. Describe how twin and adoption studies help us
differentiate hereditary and environmental influences
on human behavior.
Comparisons of identical twins, who are genetic clones,
and fraternal twins, who develop from separate eggs,
help behavior geneticists tease apart the effects of
heredity and environment. On both extraversion and
neuroticism, identical twins are much more similar than
fraternal twins. The discovery that identical twins
separated at birth show remarkable similarities also
suggests genetic influence. Indeed, separated fraternal
twins do not exhibit similarities comparable to those
of separated identical twins. However, shared genes can
translate into shared experiences.
Adoption studies enable comparisons with both genetic
and environmental relatives. Adoptees’ traits bear more
similarities to their biological parents than to their
caregiving adoptive parents. Nonetheless, the latter do
influence their children’s attitudes, values, manners,
faith, and politics. Clearly, nature and nurture shape
one’s developing personality.
Exercise: EAS Temperament Survey
Video: Segment 23 of the Scientific American Frontiers series, 2nd
ed.: Bringing up Monkey
11-4. Discuss how the relative stability of our temperament
illustrates the influence of heredity on development.
An infant’s temperament is his or her characteristic
emotional excitability and intensity. From the first
weeks of life, some babies are more relaxed and
cheerful, while others are difficult (more tense and
irritable). Still others are slow to warm up. These
differences in temperament tend to endure. For example,
the most emotionally intense preschoolers tend to be
relatively intense young adults. Compared with
fraternal twins, identical twins have more similar
temperaments, indicating that heredity may predispose
temperament differences.
Lectures: Genetic Influences on Psychological Traits; Nature and
Nurture; Gene-Environment Correlation
Feature Film: Fly Away Home and Imprinting
ActivePsych: Scientific American Frontiers, 3rd ed.: Genes and
Personality: Understanding Williams Syndrome
Instructor Video Tool Kit: 100-Years-Old and
Counting: Psychological and Biological Factors; Genes and
Personality
11-5. Explain what is meant by heritability, and give
examples of the interaction of gene and environment on
specific traits.
Heritability describes the extent to which variation
among members of a group can be attributed to genes. If
the heritability of intelligence is 50 percent, this
does not mean that one’s intelligence is 50 percent
genetic. Instead, it means that we can attribute to
genetic influence 50 percent of the observed variation
among people.
Our genes affect how our environment reacts to and
influences us. Nature enables nurture. Because of human
adaptability, most psychologically interesting traits
are expressed in particular environments. In other
words, genes are self-regulating; they can react
differently in different environments.
We are all the products of interactions between our
genetic predispositions and our surrounding
environments. For example, a stressful environment can
trigger genes that affect the production of
neurotransmitters that underlie depression. Breastfeeding boosts later intelligence only for the 90
percent of infants with a gene that assists in breaking
down fatty acids present in human milk. Similarly, a
baby who is genetically predisposed to be social and
easy going may, in contrast to one who is less so,
attract more affectionate and stimulating care and thus
develop into a warmer and more outgoing person.
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
Lecture: Designer Babies?
Videos: Module 12 of The Brain series, 2nd ed.: Huntington’s
Disease; Segment 22 of the Scientific American Frontiers series, 2nd
ed.: Bypass Genes
 Instructor Video Tool Kit: Designer Babies
 Feature Film: Gattaca and “Designer Babies”
11-6. Identify the potential uses of molecular genetics
research.
Molecular geneticists study the molecular structure and
function of genes. They seek to identify specific genes
influencing behavior. In labs worldwide, molecular
geneticists are teaming with psychologists to identify
genes that put people at risk for genetically
influenced disorders. Potentially, steps may be taken
to prevent problems before they happen. With this
benefit, however, also comes risks of labeling people
in ways that may lead to discrimination. Prenatal
screening poses hopeful possibilities but also
difficult problems as parents become able to select
their children’s traits. In China and India, where boys
are highly valued, testing for an offspring’s sex has
enabled selective abortions. Millions of parents will
select for health and perhaps for brains and beauty.
However, by “selecting” out certain traits, we may
deprive ourselves of future Handels, van Goghs,
Lincolns, and Dickinsons, who were all troubled people.
Evolutionary Psychology: Understanding Human Nature

Lectures: Evolutionary Psychology; Misunderstanding Evolutionary
Theory and Psychology
Exercises: Evolutionary Psychology; Darwinian Grandparenting
 PsychSim 5: Mind-Reading Monkeys
 Instructor Video Tool Kit: The Nature-Nurture Issue
11-7. Describe the area of psychology that interests
evolutionary psychologists, and point out some possible
effects of natural selection in the development of
human characteristics.
Evolutionary psychologists focus on what makes us so
much alike as humans. They study how natural selection
has shaped our universal behavioral tendencies.
Natural selection is the principle that, among the
range of inherited trait variations, those that lead to
increased reproduction and survival will most likely be
passed on to succeeding generations. Nature selects
beneficial variations from among the mutations (random
errors in gene replication) and the new gene
combinations produced at each human conception. During
human ancestry, genes that enable today’s capacity to
learn and adapt had survival value. Similarly, we love
the taste of fats and sweets, which once were hard to
come by but which prepared our ancestors to survive
famines. This particular natural disposition is
mismatched with today’s junk-food environment.
Lectures: Gender Differences in Sexuality; Evolutionary Theory and
Gender Differences in Motivation; Infidelity; Evolutionary
Psychology and the Coolidge Effect
Exercises: Mate Preferences; Hendrick Sexual Attitudes Scale
 PsychSim 5: Dating and Mating
Instructor Video Tool Kit: Evolutionary Psychology and Sex
Differences; Openness to Casual Sex: A Study of Men Versus Women
11-8. Identify some gender differences in sexuality and
mating preferences, and describe evolutionary
explanations for those differences.
Gender refers to the biologically and socially
influenced characteristics by which people define male
and female. One of the largest reported gender
differences is women’s greater disapproval of and
lesser willingness to engage in casual, uncommitted
sex. In comparison to women, men think more about sex,
masturbate more often, are more likely to initiate sex,
and make more sacrifices to gain sex.
Evolutionary psychologists apply the principle of
natural selection to explain women’s more relational
and men’s more recreational approaches to sex. Compared
with eggs, sperm are cheap. While a woman usually
incubates and nurses one infant at a time, a man can
spread his genes by impregnating other females. Women
most often send their genes into the future by pairing
wisely, men by pairing widely. Women increase their own
and children’s chances of survival by searching for
mates with economic resources and social status. Being
attracted to healthy, fertile-appearing partners
increases men’s chances of spreading their genes
widely.
11-9. Summarize the criticisms of evolutionary explanations
of human behaviors, and describe the
evolutionary psychologists’ responses to those
criticisms.
Critics argue that evolutionary psychologists start
with an effect (e.g., gender sexuality difference) and
work backward to propose an explanation. In addition,
much of who we are is not hard-wired. Cultural
expectations shape the genders. Still others suggest
that evolutionary explanations may undercut moral
responsibility. In response, evolutionary psychologists
point to the explanatory power of their theoretical
principles, especially those offering testable
predictions. They also note that understanding our
propensities may help us to overcome them.