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Heritability and Individual Differences Extraversion? Neuroticism? Depression? Alcoholism? Only genes are inherited Genotype – Genes Phenotype – Traits The Human Genome Contains between 30,000 and 40,000 genes All are located on 23 pairs of chromosomes The body contains roughly 100 trillion copies of the human genome The Human Genome Project Different genotypes → Same phenotype Same genotype → Different phenotype Context (other genes, cellular, etc.) is critical No specific genetic markers for personality What is Heritability? The proportion of observed variance in a group of individuals that can be explained or “accounted for” by genetic variance. What is Heritability? The proportion of phenotypic variance that is attributable to genotypic variance H of 20 = 20% GV and 80% Environmental What is Heritability? Misconceptions It can be applied to a single individual It is constant or immutable It is an absolutely precise statistic Behavioral Genetics Methods Selective Breeding Thompson Assess rat ability to traverse maze Designate rats as maze-bright or maze dull based on performance Breed maze-bright with maze-bright and maze-dull with maze dull Assess maze running ability Maze bright strain far superior to maze dull strain Cooper and Zubek Simultaneously manipulate genes and environment Genes: Obtain offspring of maze-bright and maze-dull strain Environment: Raise in normal, enriched, impoverished environment Enriched Enriched Normal Normal Restricted Restricted MazeBright 111 117 170 Maze-Dull 120 164 170 Genes matter in some environments but not others Environment can override effects of genes Behavioral Genetics Methods Twin Studies Monozygotic Twins (100% shared genes) Dizygotic Twins (50% shared genes) If MZ twins are more similar than DZ twins, this provides evidence of heritability Heritability = 2(rmz – rdz) 0.7 0.6 0.5 0.4 0.3 DZ MZ 0.2 0.1 0 -0.1 -0.2 Emotionality Activity Sociability 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 DZ MZ Crime Alcoholism Homosexuality 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 MZ MZ Verified DZ Random Pair MZ MZ Verified DZ Random Pair Trait h2 Height Weight IQ .80 .60 .50 Conservatism Religiosity TVviewing .30 .16 .20 1. Are the results valid? Do twin studies overestimate heritability? 2. What are the mechanisms that explain heritability, especially complex behaviors? Generalizability – Small, self-selected, samples Biased Ratings – Influenced by expectations MX:DZ ratio > 2:1 (Note: genetic ratio is 2:1) Possible reasons: 1. Environment MZ treated more similarly MZ interact more frequently 2. Genetic nonadditivity Twin studies with additional controls Adoption studies Twin/Adoption studies Sweden – use twin registry Measure: I, E (from Eysenck), Interaction Frequency Results: MZ > DZ for I, E, and Interaction Frequency MZ > DZ for I, E, controlling for Interaction Frequency 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 -0.05 -0.1 Adopted Parents Biological Parents Adopted Sibs Bio. Sibs Sociability Activity MZ Twins Apart Together DZ Twins Apart Together Extraversion Introversion Openness Conscientiousness Agreeableness .30 .25 .43 .15 .19 .54 .41 .51 .41 .47 .04 .28 .23 .03 .10 .06 .24 .14 .23 .11 Emotionality Activity Sociability .37 .27 .20 .49 .38 .35 .04 .00 .19 .08 .18 .19 Mean .27 .44 .11 .15 Many demonstrations of heritability, even for complex traits (e.g., crime) No gene(s) for complex phenotypes How can they be inherited? Genotype High Emotionality High Activity Ignored Punished Low self-esteem Aggressive Delinquency Phenotype Crime Selective Exposure – Genes influence types of environment to which one is exposed Differential Susceptibility – Genes influence how one reacts to environment