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Why are siblings so different
Why are siblings so different? Exploring genetic versus family influences on personality. Intro Personality Feb Two things: Siblings tend to look alike. They tend to be very different in personality. This lecture will only be covered by the final exam. It will not be on the conflict/midterm Thurs March 1st
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Questions: What role do genetics play in the development of personality traits? What aspects of the environment are important in the development of personality traits? Genes vs. Environment
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Heritability of height = 90%;
Age 18 pictures Height: .9 Weight: .9 Heritability of height = 90%; Weight?
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Richard vs Roland Early Temperament: Easy vs Difficult
Neuroticism Richard > Roland Extraversion Richard < Roland Openness Richard = Roland Agreeableness Richard > Roland Conscientiousness Richard < Roland Internal Locus of Control Personality: different from bother. Richard: More neurotic and agreeable Less extraverted (in dominance) and conscientiousness Equally open
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Heritability Quotient:
An estimate of the proportion of variability in a given characteristic that can be attributed to genetic differences between people. Can range from 0 to 100. Involves population
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Methods to explore heritability estimates for personality traits
Twin Studies Fraternal twins (DZ) share 50% Identical twins (MZ) share 100% Adoption studies Adopted sibs share 0 genes Biological sibs share 50% Combined Adoption/Twin Studies If identical twins: it doesn’t apply. Personality may be very similar. Natural experiments; For dz’s use same-sex
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If genetic endowment is important, you would expect:
1. r’s of mz twins > r of dz twins 2. r’s of bio sibs > r’s of adopted sibs. “r” is symbol for correlation; mz = monozygotic or identical twin; dz = dyzygotic or fraternal twin.
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Amazing similarities of identical twins who are raised apart
Jim Twins: 1st wife “Linda” 2nd wife “Betty” Son “James Allen” Dog “toy” Part-time cop Drive Chevy Chain-smoke same brand Drink same beer Vacation in FLA
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Identical Twins Reunited
Show Film Clip
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Major Studies Comparing Extraversion of MZ and DZ Twins
Other Big 5 Traits? MZ DZ Sweden US Canada Germany Approximately 50% heritability = .25 Most of us think that nurture is more important. Mostly think things are a results of the family upbringing. Identical = about .25 greater Heritability quotient: 2 times the difference between MZ and DZ HQ = 2 X (MZ r – DZ r)
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Twins Raised Together versus Apart
DZ together, r = .19 DZ apart, r = .14 MZ together, r = .52 MZ apart, r = .49 Extraversion: r with adoptive parent = .00, r with biological parent = .21 Can’t make the second assumption, have to look carefully at twins and then make influences of shared environment. Being raised in the same family doesn’t make people more similar. Extraversion: 0 with adoptive parent, .21 with real parents. Shared environment only accounts for only a small amount of variance.
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Two surprises in these numbers?
1. MZ's are often more than 2 X as high as DZ's. 2. There is little evidence for role of shared family environment.
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Translation into Everyday Language By McAdams:
1. Identical Twins are more similar to each other than expected. 2. Biological siblings are less similar to each other than expected.
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Non-additive genetic variance:
Genes may not influence traits in a linear, additive way, but rather combine and interact in a “configural” pattern in which all components are essential. The absence or change in any one gene can produce large quantitative changes in the result. - Biological siblings for most traits have many genes involved and to have the same personality you would need the same configuration. Because it’s not additive, sharing 50% of genes doesn’t contribute much to similarity.
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Next Question If 50% of the variation is genetic does that mean the remainder is the result of the environment, specifically parenting?
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Genetic and Environmental Variance
Shared environment: influences that operate to make family members alike Non-shared envir: influences that operate to make family members unalike At best 5% for shared environment: same house, same sister, etc. Non-shared environment: 35%, influences that make you different (friends, one is the favorite, teachers). Rest: measurement error 50% for E?
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McAdams Quote: “In teaching personality psychology for 25 years, my experience has been that students (and many colleagues) simply do not believe these results!”
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Non-Shared Environmental Influences:
Perinatal Trauma Accidental events Family Constellation Differential parental treatment Differential sibling interactions Events Outside of the Family Pregnancy: medical problems, etc. Accidental events Family constellation: can be VERY important Parents treat children differently. Very little agreement between the children. Exception: identical twins. Differential sibling interactions – how do you use siblings? Things happen outside of the family. Peer group. Family not that important after age 10.
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Birth Order Effects: Study that examined individuals within the same families First borns higher on achievement and conscientiousness; Younger siblings higher on rebelliousness and agreeableness; Paulhus et al., 1999
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Adding complexity: Extreme home environments;
More influenceable traits: Empathic concern; Delinquency; Locus of control; Not well defined (the non-shared environment). Almost all studies assume we were raised in similar environments. However, neglectful environments have effects. Meanwhile, no ideal family environment. More influencable traits: Empathic concern Delinquency Locus of control
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Questions: What role do genetics play in the development of personality traits? What aspects of the environment are important in the development of personality traits? You share 50% with your siblings but that doesn’t give you much in terms of similarity. Same parental influences but that doesn’t account for much either.
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