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Classical twin studies sometimes use self-report to determine personality – ie they ask people to rate how extraverted/conscientious/whatever they are.
These studies find that most personality traits are about 40% genetic, 60% non-shared environmental.
nurture” question is frequently investigated by twin studies, which separate interpersonal variation into three baskets: heritable, shared environmental, and non-shared environmental. Shared environmental means anything that two twins have in common – usually parents, siblings, household, and neighborhood. At least in relatively homogeneous samples (eg not split among the very rich and the very poor) studies of many different traits tend to find that ~50% of the variation is heritable and ~50% is due to non-shared environment, with the contribution of shared environment usually lower and often negligible. All these social developmentalists were so sure that the way your parents praised you or didn’t praise you, or spanked you or didn’t spank you, had long-lasting repercussions that totally shaped your adult personality.
This is typically summarized as “50% nature, 50% nurture”. The underwhelming performance of shared environment in twin studies torpedoed that whole area of study.
Two identical twins take an IQ test, one makes some lucky guesses, the other is tired, and they end up with a score difference of 5 points.
Then some random unrelated people take the test and they get the 5 point difference plus an extra 20 point difference from genuinely having different IQs.
They find that variation in personality is about 85% genetic, 15% non-shared environmental. Bob becomes a junior advertising executive at Coca-Cola, where he designs a new ad targeting young female consumers.
But at least (these scholars of social behavior could tell themselves) it provided a consolation prize.
The non-shared environment contributes 50% of variation, just as much as genes.
This gets even harder when trying to measure fuzzier constructs like criminality.
Suppose someone does a twin study on criminality and their outcome is whether a twin was ever convicted of a felony.