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Psych 2B03 - Biological Bases of Personality

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Richard B Day

Biological Bases of Personality Factors involved in Personality Genetics -> lead us to have tendencies of certain kinds of characteristics than others ENVIRONMENT Genetics Shared environment -> experiences shared by a number of people that tend to make them similar in some way (e.g. siblings) Shared Non-shared environment -> (e.g. friends, teachers, activities) different Environment experiences we have uniquely not shared by anyone else, divergent Non-Shared from shared environmental factors Environment - One of the most important factors Phenotype and Genotype - Phenotype -> observable and measurable characteristics (hair colour, eye colour, IQ, personality) - How are the phenotypes shaped by the genotypes? o Compare similarity of monozygotic (single fertilized egg that split) and dizygotic twins o Monozygotic twins share 100% of genotypes o Dizygotic share 50% of their genotypes - If genes are 100% responsible for a trait, then correlation (r) should be 1.00 for monozygotic twins even if reared together or apart (environment) o r = 0.5 for dizygotic twins - In family studies, siblings are 50% similar to other siblings and parents, 25% with uncles and aunts Problems with Studies - DZ twins confused as MZ o Sometimes when DZ twins are same sex and look similar, misclassified as MZ twins o May underestimate the importance of genetics (dilutes the MZ twin pool) - Selective placement: correlated environments in adopted twins o When twins are separated for adoptions, twins usually almost put in the same type of homes o Some similarities may not be due to genetics, more due to environmental similiarity o Leads to overestimate of power of genetics - Differences between MZ and DZ twins in environmental similarity o Always assume that MZ twins have same degree of environmental similarity and same with DZ twins in same environment o Not true -> parents treat MZ twins as exactly the same, parents emphasize individuality of DZ twins - Assortive mating: greater parent-child similarity o Assume 2 parents of any child will have no genes in common so (50% from each parent for child) o Most people choose mates with similar characteristics (may share genes in common)  May make the child more similar to a parent (can get one parent’s genes from the other  parent) Distributions and Variance - Want to quantify variability -> variance o Ơ = sum(score –mean)squared/total number - VT(total variance) = VG(genetic variance) + V Eenvironmental variance) o E.g. 9 points above the mean, 3 points due to genetics, 6 points due to environment Heritability Coefficient 2 - Heritability coefficient -> h G V TV o What proportion is due to genetic variation? o Almost always misinterpreted and mischaracterized o Ratio of genetic variance to total variance o Correlation coefficients are also ratios of variance o h = r (monozygotic twins reared apart -> estimate of heritability coefficient) mza 2  Theoretically, should be the highest estimate of h  Measure of the similarities between some characteristics in identical twins in different environments (only factor contributing to similarity should be genetics)  However don’t show their genetic effects/activity in common (determined by epigenomes)  Markers attached to some genes and cause them not to be expressed  Placed as a function in experience -> twins could have diff epigenomes 2 o h = 2rdza(double it because dizygotic twins are only half as similar) o h = 2(r-mzr dz(can use formula on either if reared together or reared apart but both sets of twins have to be in the same condition) - Applies only to groups, not individuals (applies to the population -> nomothetic) o How much of the variability in this group can be attributed to genetic variation among the members? - Varies from population to population, and over time o Changes within a population and within countries (some places have a greater genetic pool than others) - Valid only if measures used to calculate it are valid Temperament - Level of motor activity - Friendliness or good mood o How much movement they do - Adapts to changes in environment - Positive response to new object - Usual degree of energy in responding - Regularity in biological functions o High or low energy Easy Infants Difficult Infants Regular rhythms Irregular rhythms Good mood Poor mood Accepts novelty Dislikes novelty Adapts well to change Adapts poorly to change Low to moderate levels of energy High levels of energy - Wanted to believe these were built-in levels of temperament (had to be reliable, consistent, makes a difference) - Thomas Chess and Birch (1971): 70% of difficult infants, 18% of easy infants, required professional attention for personality problems o (1977, 1984) Difficult infants more at risk for anxiety disorders in childhood - Maziade et al (1990): difficult children 3-7 years are 24% of clinical population, only 10% of total population - Higher proportion of premature children are classified as difficult temperament -> usually continued to later development (but some did shift from easy to difficult and vice versa) - Parenting is an important aspect of shaping one’s initial biologically based temperament (not impossible to shift) - Plomin and Rowe (1979): studied social development of male and female twin pairs, MZ and DZ o Why are correlations for dizygotic twins so abnormally low? (no correlation)  Expected it to be half o Strong biological bases for temperament - Buss and Plomin (1984): o Emotionality -> roughly corresponds to neuroticism and stability o Activity -> typical energy in motor responding, etc. o Sociability - Where does the massive contrast effect come from? o Non-additive variance: formula used is based on the assumption that dizygotic twins share half of the genetic effects in common  Problem: not necessarily true, only true for additive genetic variance  Dominant-recessive effects: relationship b/w two  Epistatic effects: interaction between two different alleles of the same genes genes (effect of the presence of one gene on o One gene is inhibited -> dominant allele is another gene in the genome) shown instead of recessive  Dizygotic twins share 50% of additive genes in common and similarly show up in both phenotypes, but only share 25% of non-additive genes o Epigenetic effects? -> Epigenome (chemical markers) determines activity levels for genes  Chemical instructions for gene expression  Differentiates cell types in organs
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