The stepchildren of abusers, for instance, do not become abusers. The same, remarkably, is true of virtually every standard social nostrum you have ever heard. Criminals rear criminals. Divorcees rear divorcers. Problem parents rear problem children.
Obese parents rear obese children.
Having subscribed to all of these assertions during a long career of writing psychology textbooks, Judith Rich Harris suddenly began questioning them a few years ago. What she discovered appalled her. Because virtually no studies had controlled for heritability, there was no proof of causation at all in any study. Yet in each case, from behavior genetics studies, there was new, strong evidence against what Rich Harris calls 'the nurture assumption'. Studies of the divorce rate of twins, for example, reveal that genetics accounts for about half of the variation in divorce rate, non-shared environmental factors for another half and shared home environment for nothing at all.