Untangling
Nature and
Nurture
Behavior Genetics: the study
of the interaction of genes
and environment on behavior
Heritability
• The percentage of trait variation in a
group that can be attributed to
genetic differences
• Depends on the population
Heritability
Sara is very
outgoing
and
agreeable
Suzy is timid
and nonconforming
If this pair of IDENTICAL twins is my population,
what percent of the personality differences is
attributable to genetic differences between them?
Thus, the heritability is 0%.
Heritability
Joe and Hank are
not related. At
birth, both were
placed in identical
environments that
were maintained
identical until they
were 10 years old.
At 10, Joe is
outgoing and
friendly.
At 10, Hank is
withdrawn and
shy.
In this (very small) population, what is the heritability of
their personality traits? In other words, what percentage of
the difference is attributable to genes?
100%
How heritable is that trait?
Trait
Population
Creativity
Extraversion
Autism
Depression
Sleep quality
TV viewing
Religious beliefs
Religious conservatism
Divorce
Height
adolescents
adults
general
general
general
3-5 year olds
general
women
general
women
Heritability
25%
30-50%
90-95%
40-55%
35%
10-45%
0%
40%
55%
92%
http://www.peele.net/lib/genes.html The Stanton Peele Addiction Website
Twin Studies
Identical Twins
v.
Same genes,
Same environment
Fraternal Twins
Different genes,
Same environment
Therefore, greater differences between the two groups indicate
greater heritability (“Nature”). The trait might appear to have a large
genetic component.
Twin Studies
Increased risk of schizophrenia if twin
develops disease
60%
% increase
50%
40%
30%
20%
10%
0%
Fraternal twins
Identical twins
Criticism:
1) Do fraternal twins
really have equally
similar environments to
identical twins?
Differences in looks or
inborn temperament
may cause people to
treat you differently.
2) Identical twins may
share a placenta but
fraternal twins do not.
Thus, we can’t
eliminate prenatal
environments as the
cause of the differences
between the two
groups.
Twin Studies
Identical Twins Reared Together
Same genes,
Same environment
v.
Identical Twins Reared Apart
Same genes,
Different environment
Greater difference between these two groups indicates greater role of
the environment (“nurture”).
Twin Studies
Similarity of IQ Test Scores in Twins
Correlation
0.9
0.8
0.7
0.6
0.5
Identical twins reared Identical twins reared
together
apart
Criticisms:
1) Adoption agencies
try to place twins
in similar families
so the variation in
environment may
be small, and
2) There is a very
limited sample.
Adoption Studies
Is child more
like adoptive
parents…
…or biological
parents?
Adoptive Parents - Nurture
Biological Parents - Nature
Adoption Studies
Similarity of Temperament Between Adopted
Children and their Parents
0.5
Correlation Coefficient
Correlation Coefficient
Similarity of Values Between Adopted
Children and their Parents
0.4
0.3
0.2
0.1
0
Biological parents
Adoptive parents
0.6
0.5
0.4
0.3
0.2
0.1
0
Biological parents
Adoptive parents
• Note: Two related siblings that grow up in the same family are
strikingly different in personality (as I’m sure many of you can
attest to).
Your turn
Analyze the behavior genetics research
data to determine the role of nature
and nurture in various traits.