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PSYCH 2B03 (108)
Chapter 9

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McMaster University
Jennifer Ostovich

The inheritance of personality: Behavioural Genetics and Evolutionary Theory Behavioral Genetics - attempts to explain how individual differences in behaviour- personality traits- are passed from parent to child and shared by biological relatives - evolutionary psychology: explain how pattern so of behaviour that characterize all humans originate in the survival value these characteristics provided over the history of the species - connected: evolutionary approach assumes that inherited personality attributes promoting survival became more prevalent across generations, and the personality attributeswe inherited from our ancestors are the result - did you inherit your traits from your parents? Psychologically similar to siblings? - Personalitytrait: pattern of behaviour that’s relevant tomore than one situation Controversy - historic associations with a few ideas - eugentics: the belief that humanity could (and should) be improved through selective breeding o campaigns to keep inferior immigrants out,to keep sperm of Nobel Prize Winners, Hitler - cloning: it might be technologically possible to produce a complete duplicate- psychological and physical- human being - neither is actually feasible because personality is a result of complex interactions between genes and environment Calculating heritabilities - degree to which variation in phenotype (observable traits) can be attributed to variation in genotype (underlying genetic structure) - actually only concerned with 1% difference, because 99% of the human genes are identical from one individual to another  twins are similar in this 1%; MZ share all, DZ share about half - like trait psychology, behavioural genetics focuses exclusively on aspects of personality that differ from one to another o the inheritance of species-specific traits that all humans share is addressed by evolutionary biology - Eysenck Personality Questionnaire (EPQ) and California Psychological Inventory (CPI) are both commonly used in twin studies, also directly observe them in labs - Basic assumption is that if trait or a behavior is influenced by genes, then trait and behavioural scores of MZ twins are more highly correlated than those of DZ twins are  closer relatives should also be more similar on gene influenced, inherited trait than are more distant relatives - Across many traits, average correlation across MZ is 0.6, DZ= 0.4 (controlled for age/gender)  coefficient of 0.4, proportion of phenotypic variance can be explained by genetic variance is 40% - Can also do studies with siblings , relatives  estimates of 20%, which is half o Note: full siblings share 50% of variable genes, half siblings 25% - Why the difference? Effects of genes are interactive andmultiplicative rather than additive- genes interact o Although DZ twins share 50% of the genes, similarity in gene expression will be less  share only 25% of the two-way interactions among those genes o So in terms of genetic expression, identical twins will be 4 times as similar to each other than fraternal (so 20% is better estimate) - Humans don’t have thatmany more genes compared to worms and fruitflies, meaning that there has to bemore than just additive genetic effects What heritabilities tell you Genes matter - for a long time, assumed that everything determined environmentally - first law of behavioural genetics: everything is inheritable Insight into etiology - can tell you whether specific behavioural or mental disorders are part of the normal range or are pathologically distinctive - severe mental retardation is not heritable butmoderate mental retardation is  there’s some other environmental cause to severe cases then - other disorders are examples of extreme ends of the normal range o genetics of personality disorders vs. normal personality o well adjusted people vary in the degree to which they characteristically experience negative emotions but when taken to an extreme, such experiences can turn into depression o normal tendency to become absorbed in interesting activities can at the extreme be related to psychosis 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  adoptive siblings raised in the same family resemble each other with correlation ofonly 0.05- only 5% of variation in personality is due to common family environment o moreimportant is the portion of early environments that the siblings do not share (birth order, friendships outside home, other activities) - research does no tell which aspects of a child’s early environment are important- but it does suggest that whatever the key aspects may be, they are not shared across the members of the family Does the family matter? - Harris: argued that the parents and the family environmentdoesn’t matter at all - Research has shown that parental factors make an impact, but then parental style may also be confounded by the fact that parents and their children are genetically related - But when parents are taught how to be better parents, their children have both behaved better and controlled their emotions more effectively (not a shared effect since parents mayraise each child differently) - Because adoptive parents are often screened and arranged, the family environments that they foster may be more similar to each other than the environments encountered in families at large - Since all families in studies come from same culture, the extent to which they resemble each other may also underestimate the effect of family environment o When studies are corrected for family similarities, 50% variance were accounted by shared family environment - Evidence from past researchmay not be as strong o When twins and other siblings rate their personalities, they may focus on the traits that make them different from each other rather than on broader similarities that would be obvious to outside observers  may explain why shared family environment has only been shown to be important in development of aggression when measured through direct observation o Direct observation: extraversion only one which was not influenced by shared environment o Family does not play a role, was a conclusion made to quickwith limited data  should use a greater variety of data - Acknowledge/more research into the interplay between genetics and environment Nature and nurture - what % of any given trait is due to nature vs. nurture? - Heritability coefficient is the proportion of variation due to genetic influences  if there’s little variation, the lower the heritability is going to be - If a trait has high heritability, the trait may vary greatly across individuals or itmight be a trait that’s determined largely by genes - Low heritability: vary less across individuals, or it may depend less on one’s genes o The variationin the arm (1 vs. 2 arms) is produced by environmental not genetic factors  near 0 How genes affect personality - watching TV is inheritable – craving for blue light + early experiences - if one or more of your close relatives have been divorced, you are more likely to get divorced  imply that one or more genetically influenced traits are relevant to divorce (e.g. one factor may be impulsivness that’s inherited or that impulsive people have affairs) o but how they influence divorce, behavioural genetics cannot say o every personality trait thatmight affect divorce is probably heritable or these people have affairs, thus divorce may turn out to be indirectly heritable as well (Erik Turkheimer: everything is heritable) - does not explain how genes influe
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