Chapter 3
Nature, Nurture &
Human Diversity
Behavior Genetics: Predicting Individual
Differences
Behavior Geneticists study how heredity and
environment interact to influence psychological
characteristics.
Genes: Our Codes for Life
Chromosomes containing DNA (deoxyribonucleic
acid) are situated in the nucleus of a cell.
Genes: Our Codes for Life
Segments within DNA consist of genes that make
proteins to determine our development.
Genome
Genome is the set of complete instructions for
making an organism, containing all the genes in
that organism. Thus, the human genome makes
us human, and the genome for drosophila makes
it a common house fly.
Twin Biology
Studying the effects of heredity and
environment on two sets of twins, identical and
fraternal, has come in handy.
Twins Studies
Behavior geneticists’ study identical and fraternal
twins to determine the role of genes in behavior,
personality, etc.
Famous Ongoing Identical Twin Study:
 University of Minnesota Twin Family Study
 Founded by Thomas Bouchard & Currently led by David Lykken
 Basic Findings:
 Identical Twins tend to share similar personality traits, levels of
intelligence, likes/dislikes/fears, brainwaves and heartbeat patterns
 Identical twins separated at birth and raised separately also tend to
show such similar traits
 Most Famous Example of subjects: Jim Lewis and Jim Springer
 Both had been married twice
 Both had sons named James Alan/James Allan
 Both smoked the same brand of cigarettes and drank the same kind of beer
 Both worked as part-time sheriffs
 Both suffered stress-related migraines
 Both bit their fingernails
 Both built a circular bench around a tree in their front yards
 Both had dogs named “toy”
 Scored similarly on intelligence and personality tests
 Had similar speech patterns
A Note of Caution about Separated
Twins Studies:
Critics of separated twin studies note that such
similarities
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can be found between strangers.
can be a result of placement of separated twins into
similar families
are based on a few examples as opposed to a wide
range of examples
In response, the separated twin researchers
point out that differences between fraternal
twins are greater than identical twins.
Adoption Studies
 Used to study the effects of nurture/environment
on behavior, personality, etc.
 Basic findings:
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Adoptees tend to have more similar personality traits to
their birth parents than to their adoptive families
Furthermore, siblings (biologically related or not) do not
tend to show the same personality traits.
But, children (adopted or not) tend to show similar
attitudes, values, morals, political tendencies as do the
parents who raised them
Adopted children tend to be very well-adjusted people!
Temperament Studies
Temperament refers to a person’s stable
emotional reactivity and intensity.
Identical twins express similar temperaments,
suggesting heredity predisposes temperament.
Heritability
Heritability refers to the extent to which the
differences among people are attributable to genes.
In other words, if the environment is equal,
then the results between two individuals would
be more attributable to genetic factors (high
heritability).
Or, if the environment is vastly different, but
the genetic factors similar, the results would be
due to low heritability
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.
Nature and Nurture
Some human traits are fixed, such as having
two eyes. However, most psychological traits
can change with environmental experience.
Genes provide choices for the organism to
change its form or traits when environmental
variables change. Therefore, genes are pliable or
self-regulating. In other words, genes react to the
environmental conditions.
Gene-Environment Interaction
Genes can influence traits which affect
responses, and environment can affect gene
activity. Genes and environment interact.
A genetic predisposition that makes a child restless
and hyperactive evokes an angry response from
his parents. A stressful environment can trigger
genes to manufacture neurotransmitters leading
to depression.