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, children’s IQ was significantly correlated with both biological mother (.23) and adoptive mother (.13) • At time 2, children’s 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