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