Title: Genetic and Environmental Influences on Personality
1Genetic and Environmental Influences on
Personality
2Major Points
- Many individual differences in personality are
influenced by genetic factors - The relative contribution of genetic vs.
environmental factors depends on the aspect of
personality that is being studied
3Major Points
- We will examine research findings related to
- big five personality traits
- intelligence
- attitudes
- sexual orientation
4Heritability
- Proportion of variance in some trait that is due
to genetic factors - Estimate of the influence of genotypes on
phenotypes in a particular population - Genotype collection of genes determined for
each human at conception - Phenotype measurable characteristics of persons
- Heritability estimates range from .00 to 1.00
5Important Points
- Heritability is not a fixed property of a
phenotype - If environment is homogenous, influence of
genotypes on phenotypes will increase
(heritability will be higher) - As environmental variations increase, influence
of genotypes on phenotypes will decrease
(heritability will be lower) - Even highly heritable traits can be modified by
environment
6Common Research Methods
- Twin studies
- Can compare correlations between MZ twins to
correlations between DZ twins - Comparisons based on equal-environments
assumption (which may be wrong) - Adoption and Family studies
- Can compare correlations between biologically
related and biologically unrelated family members
7Types of Genetic Influences
8Additive Genetic Influences
- Cause individuals who are genetically similar to
develop similar phenotypes - Cause genetically similar individuals to resemble
each other - If trait influenced solely by additive genetic
influences - MZ twins (raised together or apart) would
correlate 1.00 - DZ twins and other siblings (raised together or
apart) would correlate .50 - children and parents would correlate .50
9Nonadditive Genetic Influences
- Lead individuals who are genetically similar (but
not identical) to differ with respect to
phenotype - Cause children to differ from parents, and
siblings to differ from each other for purely
genetic reasons - If phenotype influenced solely by nonadditive
genetic influences - MZ twins (raised together or apart) would
correlate 1.00 - DZ twins and other siblings (raised together or
apart) would correlate no higher than .25
10Nonadditive Genetic Influences (continued)
- Nonadditive genetic influences are caused by
- Dominance
- Epistasis
11Dominance
- If gene is recessive, it will not influence
phenotype in presence of dominant gene - Example phenylketonuria
- If child inherits one recessive gene from each
parent, child will have PK - Child will be similar (but not identical) to
parents in genotype, but not phenotype
12Epistasis
- Occurs when genes in different locations interact
with each other - Can end up with persons who are genetically
similar, but phenotypically different
13A Scenario
- Imagine trait is influenced by 3 genes in
different locations - Person 1 has A, B, and C.
- Person 2 has A and B.
- Person 3 has none.
- Suppose individuals with all 3 genes receive high
score on phenotype, and low score if any of 3 are
missing - Persons 1 and 2 would differ with respect to
phenotype, even though they are genetically
similar. - Persons 2 and 3 would be similar with respect to
phenotype, even though they are genetically
different
14Types of Environmental Influences
- Shared environmental influences
- Nonshared environmental influences
15Shared Environmental Influences
- Cause individuals reared in the same family to
resemble each other with respect to phenotype - Examples books in home, presence/absence of
TV, quality and quantity of food, parents
values/attitudes, school, church - If phenotype influenced solely by shared
environmental influences - MZ twins reared together would correlate 1.00
- DZ twins reared together would correlated 1.00
- Siblings reared together (whether biologically
related or not) would correlate 1.00
16Nonshared Environmental Influences
- Cause individuals reared in the same family to
differ from other with respect to phenotype - Examples treated differently by parents,
different friends, different teachers, different
lovers, some go to camp - If trait influenced solely by nonshared
environmental influences - MZ twins (whether reared together or apart) would
correlate 0 - DZ twins (whether reared together or apart) would
correlate 0 - Siblings (whether reared together or apart) would
correlate 0
17Personality Traits
- Heritability of major personality traits is about
50 - 50 of individual differences are due to genetic
factors - 50 of individual differences are due to
environmental factors
18Twin Data for Extraversion
- Swedish study (13,000 pairs of twins)
- MZ twins r .51
- DZ twins r .21
- Australian study (3,000 pairs of twins)
- MZ twins r .52
- DZ twins r .17
19Twin Data for Neuroticism
- Swedish study (13,000 pairs of twins)
- MZ twins r .50
- DZ twins r .23
- Australian study (3,000 pairs of twins)
- MZ twins r .50
- DZ twins r .23
20Importance of Environmental Factors in Personality
- 50 of individual differences in major
personality traits are accounted for by
environmental factors - Nonshared environmental factors are more
important than shared - Average correlation between adoptive siblings
raised together is .05
21Intelligence
- Texas Adoption Study (large, well controlled)
- Results showed
- At time 1, childrens IQ was significantly
correlated with both biological mother (.23) and
adoptive mother (.13) - At time 2, childrens IQ was significantly
correlated only with biological mother (.26).
Correlation with adoptive mother was .05.
22Other Research
- Near 0 correlations between biologically
unrelated siblings - Twin data
- MZ reared together r .76
- MZ reared apart r .77
- DZ reared together r .22
- DZ reared apart r .32
23Intelligence and Personality Compared
- Heritability is higher for IQ than personality
- Example Correlations for MZ twins are higher
for IQ (.76) than personality (.50) - Genetic influences are stronger for IQ than
personality - Shared family environment is not an important
influence on either IQ or personality
24Would you resemble an MZ twin raised apart from
you?
- Similar in intelligence (.77)
- Somewhat similar in personality (.51)
25Would you resemble an adoptive sibling raised
with you?
- Not in intelligence ( .05)
- Not in personality (.04)
26Attitudes
- In general, the relative importance of genes vs.
attitudes depends on the attitude being studied - heritability estimates range from 0-60
- Many twin studies show higher correlations for MZ
than DZ twins - For religiosity among males, MZ .66, DZ .51
- For prejudice among females, MZ .61, DZ .58
27Attitudes
- Correlations for DZ twins are more than half the
value of correlations for MZ twins - May reflect fact that shared family environments
influence attitudes - May reflect effects of assortative mating
- Tendency of individuals to choose mates who are
similar to them - End up with siblings who share more than 50 of
their genes
28Sexual Orientation
- Previous research on the heritability of
homosexuality has yielded estimates ranging from
30-70 - concordance rates for MZ twins range from 40-50
- Previous research may have been flawed
- subjects recruited from advertisements in gay and
lesbian publications - subjects may be more likely to answer ad if twin
is also gay
29Bailey et al. (2000)
- Method
- analyzed data from sample of 25,000 twin pairs in
Australia - randomly selected 2,000 twin pairs
- twins filled out questionnaires about childhood
participation in sex-stereotyped activities and
adult sexual orientation and activity
30Bailey et al. (2000)
- Results
- 3 of men and 1 of women were exclusively
homosexual - concordance rates for MZ twins were
- 20 for men
- 24 for women
- estimate of direct genetic contribution
(independent of environmental effects) was not
statistically significant - childhood gender nonconformity did show
significant heritability (50 for men 37 for
women)
31Bailey et al. (2000)
- Interpretation
- childhood gender nonconformity may be the
inherited aspect of homosexuality - one theory early gender nonconformity causes
children to feel different from children of their
own sex, and as a result, to be attracted to
people who are different from themselves (even
though they are of the same gender)
32Do parents influence the personality
characteristics of their children?
- On the one hand
- Shared family influence does not lead individuals
to be similar with respect to the big five
personality traits or intelligence - correlations for biologically unrelated family
members are near 0
33- On the other hand
- research comparing different child-training and
parenting practices indicates long-lasting
differential effects - certain types of adult personality
characteristics are linked to parental practices - antisocial personality disorder is linked to
harsh, inconsistent parental discipline and
father-absent homes
34- cross-fostering experiments with animals show the
important effects of parenting - genetically excitable, reactive infant monkeys
raised by calm mothers develop normal peer
relationships and normal reactions to stress - genetically excitable, reactive infant monkeys
raised by excitable, easily distressed mothers
develop problematic peer relationships and are
quite vulnerable to stress
35Summary
- Personality
- Genetic and environmental influences contribute
about equally to individual differences in
personality - Intelligence
- Genetic influences are more important than
environmental influences in contributing to
individual differences in IQ
36Summary
- Attitudes
- Genetic and environmental influences both
contribute to individual differences in attitudes - The relative contribution of each depends on the
attitude being studied - Sexual Orientation
- Genes probably contribute to sexual orientation,
but not in the way we first thought