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Chapter 9

Chapter 9 The Inheritance of Personality - Textbook Notes

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Richard B Day

Psych 2B03 2012 Part III: The Mind and The Body: Biological Approaches to Personality Chapter 9: The Inheritance of Personality: Behavioural Genetics and Evolutionary Theory  Two biologically based approaches that consider the ways personality might be inherited (how characteristic patterns of behaviour might be encoded on genes and passed from parents to children across generations) 1. Behavioural Genetics – attempts to explain how individual differences in behaviour (personality traits) are passed from parent to child and shared by biological relatives 2. Evolutionary Psychology – attempts to explain how patterns of behaviour that characterize all humans originated in the survival value these characteristics provided over the history of the species Behavioural Genetics  This field of research examines the way inherited biological material (the genes) can influence broad patterns of behaviour  Personality Trait – a pattern of behaviour relevant to more than one situation Controversy  Behavioural Genetics have a historic association with notorious ideas o Eugenics – the belief that humanity could be improved through selective breeding  Has led to activities ranging from campaigns to keep “inferior” immigrants out of some countries, to attempts to set up sperm banks stocked with deposits from winners of the Nobel Prize o Cloning – the belief that it might be technologically possible to produce a complete duplicate (psychological, physical) of a human being  Neither of these ideas are very feasible; if personality is the result of a complex interaction between genes and environment, the chances of being able to breed people to specifications or duplicate individuals are slim  The real contributing of behavioural genetics is the way it expands our understanding of the sources of personality to include bases in both genes and the environment Calculating Heritabilities  Basic Question – the degree to which variation in the phenotype (observable traits of a person) can be attributed to variation in the genotype  More than 99% of human genes are identical from one individual to another (also share 98% with chimpanzees)  Behavioural genetics focus on the 1% that differs; focuses on aspects of personality that differ from one individual to another  Compare similarities in personality between individuals who are and are not genetically related, or who are related to each other to different degrees  Twin Studies o Monozygotic – MZ, identical; come from the splitting of a single fertilized egg; genetically identical o Dizygotic – DZ, fraternal; two eggs fertilized by two different sperm; no more genetically identical than any two siblings o Observe twins o Compute the correlation coefficient  Assumption – if a trait of a behaviour is influenced by genes, then the trait and behavioural scores of MZ twins ought to be more highly correlated than scores of DZ twins  Heritability coefficient is computed to reflect this influence  Heritability quotient = mzr dz )  Average correlation across many traits when age and gender is controlled is 0.60 for MZ, 0.40 for DZ  2(0.60-0.40) = 0.40; interpreted to mean that the proportion of phenotypic (behavioural) variance than can be explained by genetic variance is 40%  Non-Twin Studies; alternative ways of calculating similarities in personality – other relatives vary in degree to which they share genes o Children share ~50% of variable genes with biological parents and none with their adopted parents o Full siblings share ~50% of genes o Half-siblings (share one parent) share ~25% of genes o For most traits the estimates of heritability gathered from non-twin studies are ~20%  Effects of genes are interactive and multiplicative (rather than additive) – accounts for difference in heritability between twin and non-twin studies o Twin studies calculations assumes that because DZ twins share half the variable genes that MZ twins do, they are half as similar in genetic expression 1 Psych 2B03 2012  If genes act not just by independently adding their effects together, but also interacting with one another, then DZ twins similarity in genetic expressions will be less than 50%  Though they share 50 percent of the genes, they share only 25% of the two-way interactions among those genes  Identical twins will then be 4x as similar to each other than fraternal twins (instead of only 2x as similar)  The 20% figure for heritability may be more accurate than the 040% figure o Vast difference between humans and other species cannot be accounted for by adding up genetic effects – reason to conclude that genes interact with each other  Human Genome – 25 000 genes  Fruit Fly genome – 13 000-14000  Roundworm genome – 19 000 What Heritabilities Tell You  Genes Matter  Insight into Etiology o Can tell you whether specific behavioural or mental disorders are part of the normal range or are pathologically `distinctive o Severe mental retardation is not heritable, but moderate mental retardation is  Severe mental retardation – IQ below 50, when 100 is average  Average IQ of siblings of severely mentally retarded child is 103  Moderate mental retardation – IQ between 50-69  Average IF of siblings of a moderately retarded child is 85  Implies that the cause of severe mental retardation comes from the environment; infection during birth, birth trauma o Genetic roots at extreme ends of the normal range  Insight into Effects of the Environment o Growing up together in the same home does not tend to make children similar to each other; personality traits of adoptive siblings raised in the same family resemble each other with a correlation of only 0.05 due to common family environment (typically measured in Big Five traits) o Early environments that siblings do not share seem to have a greater influence on personality  Effects of birth order, friendships outside the home, other outside interests and activities Does The Family Matter?  Neighborhood, home atmosphere, income, nutrition, parents styles of child rearing, presence/absence of one or both parents  Judith Rich Harris – parents do not have any important long-term effects on the development of their child’s personality o Denies all prior fundamental assumptions that have guided the study of development o Implies that people do not need to try to be good parents; no reason to remove children from abusive homes  Research in developmental psychology have documented that the effects of child rearing, family environment and social class on personality  Research on how parental styles affect children have been confounded by the fact that parents and children are genetically related  When parents are taught to be better parents, children behave better and control emotions more effectively – important way in which family environment matters  Technical complication – adoptive families are screened, chosen and arranged by social service agencies; the family environments they foster may be more similar to each other than are the environments encountered in families at large  All families in typical heritability studies come from the same culture – to the extent that families in the study resemble each other, effect of family environment on personality tend to be underestimated o The range has been restricted on the “family environment” variable, lowering its change of demonstrating an effect on any other variable, lowering its change of demonstrating an effect on any other variable o According to one recent reanalysis, when heritability calculations are corrected for the similarities among families, it appears that up to 50% of the variance in individual differences such as IQ are accounted for by the shared family environments  Several developmental outcomes (juvenile delinquency, love styles, aggression) – standard methods of behavioural genetics are affected by shared family environment  Results may depend on methods used o Eg/ twins and other siblings rated own personalities; may focus on broader similarities that would be obvious to outside observers  may be why shared family environment has been shown to be important in development of 2 Psych 2B03 2012 aggression only when aggression is measured through direct observation (B data) rather than questionnaires (S data) o Studies of other traits – found that extraversion was the only trait that is not influenced by shared environment; 14 other traits were found to be influenced by shared environment  Two implications o Widely advertised conclusion that family environment is unimportant for personality development was reached too quickly, on the basis of limited data  Behavioural genetics focused on S data and found little similarity across siblings raised together; but when personality is based on B data, shared family environments show important effects o Personality research can employ many kinds of data, and they all should be used; conclusions based on one kind of data are risky, conclusions based on consistent results across several kinds of data are much more likely to hold up in the long run  Scientific evidence is that parents matter Nature and Nurture  Heritability formula does not work for a lot of shared traits  The less a trait varies across individuals, the lower its heritability is likely to be o If a given trait has a high heritability, two situations are possible  Trait might vary greatly across individuals  Trait might be determined largely by genes How Genes Affect Personality  Statistically: o Television watching is heritable  Sensation seeking, lethargy, craving blue light may have genetic component, which interacts with biological development and early experience – causes people to watch a lot of TV o Divorce is heritable  Impulsivity, homosexuality, alcoholism, depression (etc, other traits that cause divorce) may be heritable  Molecular Genetics o New research on DNA has begun to look into how specific genes influence life outcomes o Association Method – tries to determined whether differences in a trait correlate with differences in a particular gene across individuals o Genetic basis of homosexuality in males  Researchers found a group of homosexuals who were related to various degrees  Used microbiological techniques to identify a gene on the X chromosome that most of the homosexuals shared but was not present in heterosexual members of the same family  Concluded that this genetic similarity was one basis of homosexuality o Relationship between traits associated with behavioural and emotional control and a gene called DRD , which 4 affects the development of dopamine receptors  Dopamine – part of the brain system that responds to reward; play a broad role in the control and regulation of behaviour and bodily movement  Theory that shortage in dopamine (or inability to respond to it) leads people to crave extra stimulation to the point of engaging in risky behaviour  People with dopamine shortages caused by Parkinson’s disease develop tremors and may eventually lose muscular control altogether  Different forms of DRD 4re associated with variations in sensation seeking; gene might be basis of sensation seeking via its effect on dopaminergic systems  Variations in DRD 4orrelated with participants scores on tests of novelty seeking  DRD i4 also associated with the risk for attention deficit hyperactivity disorder (ADHD) o Serotonin has been blamed for a wide variety of emotional disorders ranging from depression to anxiety and social phobias  Drugs (eg/ SSRIs) that increase the level of serotonin transporter protein 5-HTT appears to be particularly important in the development of neuroticism (trait relevant to anxiety and overreaction to stress as well as depression)  Alleles (different forms of the same genes)  5-HTT gene has a short and long allele, based on the shape of the chromosomal structure; people with short allele score higher on measures of neuroticism 3 Psych 2B03 2012  Amygdala in people with short allele show stronger responses, as viewed through fMRI images, PET scans and other imaging techniques, to fearful and unpleasant stimuli such as pictures of frightening- looking faces, accident victims mutilates bodies and polluted scenery  5-HTT gene appears to regulate the degree to which the amygdala and the prefrontal cortex work together, which may offer an important clue to the brain structure of depression  Procession of the short allele correlated with how much the amygdala of people who suffer from social phobias respond when giving a public speech  Prevalence of the short allele vary across cultural groups – present in more than 75% of Japanese people, which is more than double that of Caucasians  May be reason why Asian cultures emphasize cooperation and avoiding conflict over the kind of individualistic striving said to be characteristic of Western cultures  Limitation – findings do not apply to everyone Gene-Environment Interactions  Genes cannot cause anybody to do anything – genotype only indirectly affects behavioural phenotype by influencing biological structure and physiology as they develop within an environment  Genes can influence the development of behaviour only in people who live in some kind of environment; without an environment, there would be no behaviour o Reverse is also true – without a person built by genes, no behaviour can occur  Genes and the environment interact – neither can do anything without the other  Environment can affect heritability – o Eg/ Environment where every child received adequate nutrition, variance in height will be genetically controlled  Tall parents = tall children, short parents = short children  Heritability coefficient will be large o Eg/ environment where food is scarce and children have poor nutrition, variance in height will be controlled by environment  Well fed children = tall, poorly fed children = short  Heritability coefficient will be small  Several interactions between genes and environment o Eg/ boy is short due to genetics, he is teased because he is short which affects his personality – genes and environment affect personality  How people choose environments o Eg/ Inherited tendency to seek sensation; may choose dangerous drugs; harms health which determines environment  Same environment can affect individuals differently o Eg/ stressful environment may lead a genetically predisposed individual to develop mental illness but leave individuals without predisposition psychologically unscathed  Avshalom Caspi and Terrie Moffitt o Study of a group of children examined why stressful experiences lead to depression later in life for some people but not others  Assessed the degree to which participants experienced stresses such as unemployment, financial setbacks, housing problems, health challenges, relationship problems between ages 21-26, then whether they experienced depression at the end of this period  Found that people who had the short 5-HHT allele were relatively likely to experience depression following stress  No difference in outcome between those with the long allele and those with short if they did not suffer from stress o Study examined why maltreated children become delinquents or adult criminals, while others do not  Targeted gene is part of X chromosome and has been shown to affect the enzyme MAOA (monoamine oxidase A) which influences functioning of a range of neurotransmitters (including norepinephrine, serotonin and d
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