PSYB30 PERSONALITY Prof. Marc A. Fournier Week 6 Heritability & Continuity Week 6 in Perspective The Origin Theories—Evolution & Socialization Week 1. Introduction Week 2. Human Evolution Week 3. Gender, Society, & Culture Level I. Personality Traits / Dispositions Week 4. History & Controversy of Trait Concepts Week 5. Contemporary Trait Taxonomies Week 6. Heritability & Continuity ◄ Level II. Characteristic Adaptations Week 7. Goals & Strivings Week 8. The Self & Social Cognition Week 9. Stages of Life-Span Development Level III. Integrative Life Stories Week 10. Life Scripts & Life Stories Week 11. Myth & Narrative Week 12. Conclusion Overview of Week 6 Lectures Continuity (& Coherence) Heritability (& the Environment) The Up Series Overview of Part I Continuity of Personality Do individual differences in some attribute predict individual differences in that same attribute at some later point in time? (X1 → X2) Coherence of Personality Do individual differences in some attribute predict individual differences in some other attribute at some later point in time? (X1 → Y2) Continuity of Personality Consistency of individual differences—i.e., relative position or rank order—within a population of individuals over time Differential continuity is independent of normative changes in trait levels e.g., height, intelligence Continuity of Personality How early in life can we see personality consistency? And when (if ever) does personality become set in stone? (Freud) by age 5 or 6 (James) by age 30 Roberts & DelVecchio (2000): Meta-analyzed findings from 152 longitudinal studies involving a total of 3,217 consistency coefficients and 50,207 participants Continuity of Personality Roberts & DelVecchio (2000)—Continued: Consistency increased as a function of age. Holding time interval constant at 6.7 years, consistency coefficients equaled: .31 in infancy .43 in middle childhood .54 during the college years .64 during one’s 30s .74 during one’s 50s Continuity of Personality Roberts & DelVecchio (2000)—Continued: Consistency decreased as a function of time interval. Holding age constant at 20 years, consistency coefficients equaled: .55 over a 1-year period .52 over a 5-year period .49 over a 10-year period .41 over a 20-year period .25 over a 40-year period Continuity of Personality Roberts & DelVecchio (2000)—Conclusion: The differential (or rank order) continuity of traits emerges in childhood and peaks around the age of 50, but not at a level high enough to conclude that our traits at some point become set in stone Coherence of Personality The inferred consistency of some underlying attribute despite observed change in its behavioral expression The investigator must have a theory of how the diversity of behavior observed across the life course can be said to belong to the same equivalence class Coherence of Personality Berkeley Guidance Study A study initiated in 1928 with every third birth in Berkeley, CA (original N = 214 subjects) over a period of one and a half years Mothers were interviewed when subjects were ages 8-10; life outcomes were collected when subjects were ages 30 and 40 Coherence of Personality Moving Against the World: Life-Course Patterns of Explosive Children Caspi, Elder, & Bem (1987): frequency & severity of temper tantrums in late childhood predicted: In men (from middle-class homes) — deterioration in SES, erratic work lives, and divorce In women — marriage to men of lower SES, divorce, and inadequate parenting Summary of Part I There is considerable evidence of trait continuity across the life course The continuity of traits emerges early in life, and peaks around age 50 Although our underlying traits are theorized to remain constant, their correlates can vary across the life course Overview of Part II Methods of Behavioral Genetic Research Twin Studies & Adoption Studies Strengths & Limitations The Jim Twins Jim Springer & Jim Lewis were identical twins separated at birth, who met for the 1st time at 39 years of age Their striking similarities in terms of personality and life outcomes raise the intriguing question— to what extent is personality genetically determined? The Human Genome “Genome” refers to the complete set of genes that an organism possesses The human genome contains somewhere b/n 20,000 and 25,000 genes Genes are located on 23 chromosomal pairs, 1 pair from each parent Heritability The proportion of OBSERVED VARIANCE in a GROUP of INDIVIDUALS that can be accounted for by GENETIC VARIANCE Misconceptions About Heritability Heritability estimates … Are precise (no, they aren’t!) Stay the same over time (no, they don’t!) Can be applied to a single person (no, they can’t!) The Nature-Nurture Debate Are personality characteristics determined by our genes or by the environment? At the level of the individual, there is no debate between genes and environments Only at the population level can we disentangle genetic and environmental effects Family Studies Correlate the degree of genetic overlap with the degree of personality similarity Capitalize on the fact that there are known degrees of genetic overlap among family members Confound: family members who share more genes in common may also share more similar environments Twin Studies Are identical (MZ) twins more similar to each other than fraternal (DZ) twins? Heritability = 2(rmz – rdz) Assumptions of twin studies: Equal Environments, Representativeness Adoption Studies To whom are adopted children more similar — their biological or adoptive parents? Adoption studies circumvent the assumption of equal environments Potential problems: Representativeness, Selective Placement Summary of Part II Behavioral genetic researchers have developed a range of methods to partition the genetic and environmental effects on personality Adoption studies are more powerful than twin studies, and (although rare) adoption studies with twins are the most powerful of all Overview of Part III Findings from Behavioral Genetic Research: Twin Studies & Adoption Studies The Three Puzzles Findings from Twin Studies Trait MZ Corr DZ Corr Heritability N .41 .18 .46 E .55 .23 .64 A .41 .26 .30 C .37 .27 .20 O .58 .21 .74 Note. 123 MZ twin pairs & 127 DZ twin pairs. (Jang, Livesley, & Vernon, 1996) Findings from Adoption Studies Less genetic influence on personality: e.g., upper-limit of heritability estimates ≈ 30% for extraversion, 15% for neuroticism Less environmental influence on personality: Correlations between adoptive parents and their adopted children tend to be around zero The 1st Puzzle Identical twins are more similar to each other than expected (from twin studies) i.e., more than twice as similar as fraternal twins Biological sibs are less similar to each other than expected (from adoption studies) i.e., slightly more similar than adoptive siblings Nonadditive Genetic Effects In nonlinear/nonadditive terms, A heredity estimate of 100% is more than twice as large as an estimate of 50% A heredity estimate of 50% is in fact not much larger than an estimate of 0% The 2nd Puzzle If the heritability of personality traits is ≈ 40%, what accounts for the remaining 60% of the trait variance? Measurement error ≈ 20%; so, the remaining 40% of the trait variance should be environmental in origin However, adoption studies suggest that environments do not render adoptive parents and adopted children similar to each other Shared & Nonshared Environments Shared Effects: Those environmental influences that operate to make family members alike Nonshared Effects: Those environmental influences that operate to make family members unalike The 3rd Puzzle Indices of the environment show as much genetic influence as indices of personality How can ratings of family environment from twins reared apart show genetic influence, when the twins rate different families? How Genes Shape Environments Passive Genotype-Environment Correlation: Children “inherit” contexts from their parents that are correlated with their genetic propensities Reactive Genotype-Environment Correlation: Children evoke reactions from their parents that are correlated with their genetic propensities Active Genotype-Environment Correlation: Children select and construct experiences that are correlated with their genetic propensities Summary of Part III Summary of Week 6 Lectures There is considerable evidence for the continuity and coherence of personality traits across the life course. Continuity emerges early in life, and peaks around age 50. Traits reflect both genetic and environmental determinants, though the effect of the environment appears to be one that produces fewer trait similarities than trait differences. Environments themselves appear to be heritable, in part because the genetic differences b/n individuals correlate with the environments to which they are exposed.