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Genetic and Environmental Influences on Personality

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Title: Genetic and Environmental Influences on Personality


1
Genetic and Environmental Influences on
Personality
2
Major 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

3
Major Points
  • We will examine research findings related to
  • big five personality traits
  • intelligence
  • attitudes
  • sexual orientation

4
Heritability
  • 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

5
Important 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

6
Common 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

7
Types of Genetic Influences
  • Additive
  • Nonadditive

8
Additive 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

9
Nonadditive 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

10
Nonadditive Genetic Influences (continued)
  • Nonadditive genetic influences are caused by
  • Dominance
  • Epistasis

11
Dominance
  • 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

12
Epistasis
  • Occurs when genes in different locations interact
    with each other
  • Can end up with persons who are genetically
    similar, but phenotypically different

13
A 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

14
Types of Environmental Influences
  • Shared environmental influences
  • Nonshared environmental influences

15
Shared 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

16
Nonshared 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

17
Personality 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

18
Twin 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

19
Twin 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

20
Importance 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

21
Intelligence
  • 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.

22
Other 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

23
Intelligence 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

24
Would you resemble an MZ twin raised apart from
you?
  • Similar in intelligence (.77)
  • Somewhat similar in personality (.51)

25
Would you resemble an adoptive sibling raised
with you?
  • Not in intelligence ( .05)
  • Not in personality (.04)

26
Attitudes
  • 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

27
Attitudes
  • 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

28
Sexual 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

29
Bailey 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

30
Bailey 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)

31
Bailey 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)

32
Do 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

35
Summary
  • 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

36
Summary
  • 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
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