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chapter 9.docx

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McMaster University
Richard B Day

Psych 2B03: Theories of Personality Chapter 9: The Inheritance of Personality – Behavioural Genetics and Evolutionary Theory Behavioural Genetics - Controversy:  Eugenics: the belief that humanity could be improved through selective breeding  Cloning: the belief that it might be technologically possible to produce a complete duplicate  Modern behavioural geneticists are generally quick to dissociate themselves from these ideas  The real contribution of behavioural genetics is the way it expands out understanding of the sources of personality to include bases in both genes and the environment - Calculating heritabilities:  Compare similarities in personality between individuals who are and are not genetically related  Concerns the degree to which variation in the phenotype can be attributed to variation in the genotype  Twin studies  Heritability quotient = (rMZ– rDZ × 2  According to twin studies, the average heritability of many traits is about .40  For most traits the estimates of heritability garnered from nontwin studies are about 20%, or half the average heritability estimated from twin studies  Effects of genes are interactive and multiplicative rather than additive  Although DZ twins share 50% of the genes, they share only 25% of the two-way interactions among those genes  The vast differences between humans and these other species, not to mention the vast diversity among humans, cannot be accounted for by adding up genetic effects - What heritabilities tell you:  Genes matter:  Not all personality comes from experience  Insight into etiology:  Heritabilities can tell you whether specific behaviour or mental disorders are part of the normal range or are pathologically distinctive  Severe mental retardation is not heritable, moderate retardation runs in families  Other psychological problems appear to be examples of extreme ends of the normal range  Insight into effects of the environment:  Provide a window into how the early environment does – or does not – operate in shaping personality development  Growing up together in the same home does not tend to make children similar to each other  Early environment that siblings do not share: the effects of birth order, friendships outside the home, and other outside interests and activities - Does the family matter?  Aspects of the family environment (neighbourhood, home atmosphere, income, nutrition, parents’ style of child rearing, and the presence or absence of one or both parents) are not important in determining the king of adult each child grows up to be  Many decades of research in developmental psychology have documented the effects of child rearing, family environment, and even social class on personality: some of the effects that psychologists have attributed to the way parents raise their children may instead be due to the genes that parents share with their children  Experiments have shown that when mothers and fathers are taught how to be better parents, their children both behav
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