But then came the studies of twins. In the 1980s, when scientists began analyzing large databases that allowed them to compare identical twins (who share all of their genes, plus, usually, their prenatal and childhood environments) to same-sex fraternal twins (who share half of their genes, plus their prenatal and childhood environments), they found that the identical twins were more similar on just about everything.12 And what’s more, identical twins reared in separate households (because of adoption) usually turn out to be very similar, whereas unrelated children reared together (because of
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