Identical Twin A becomes a small-time cocaine dealer in a back alley in West Philly, sells to an undercover cop, and ends up in jail.
These studies find that most personality traits are about 40% genetic, 60% non-shared environmental.Science reporters read the study finding that much of the variation in income is non-shared environmental, and conclude that despite their identical genes, there must be deep and mysterious differences in Bob and Rob’s abilities and business acumen.They speculate that Rob had a very inspirational teacher in school who pushed him to achieve greatness, and Bob must have fallen in with a bad peer group who didn’t value hard work.This shows up as “variation in criminality is due to non-shared environment”.Riemann and Kandler (h/t Jay Man) run a study which is an excellent demonstration of this.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.This depends partly on whether the twin is actually the sort of person with criminal tendencies – but also partly on whether a policeman happened to be in the area to catch them, whether their lawyer happened to be good enough to get them off, whether their judge was feeling merciful that day, et cetera.Imagine a world where criminality is entirely genetic.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.