So let me just point to this review of Personality: What Makes Us the Way We Are (Oxford) by Daniel Nettle. The key passage is:
Behavioural-genetic studies have consistently found that the heritability of personality traits, whether the Big Five or one of many others from aggressiveness to happiness, is around 50 per cent. This means that within a group of people, about 50 per cent of the variation in such traits is attributable to genetic differences among the individuals in the group. Most people have assumed that the other 50 per cent comes from the “shared environment” of the home: parental child-rearing methods and the experiences the child shares with siblings and parents. If it did, studies should find a strong correlation between the personality traits of adopted children and those of their adoptive parents. In fact, the correlation is weak to nonexistent. This means that when children resemble their parents and grandparents temperamentally, it is because they share genes with these relatives, not experiences. What, then, is going on in the “unshared environment”, the other half of the influences that “make you the way you are”? Put simply, Nettle argues, we don’t know.
Although this is hardly the explanation I’ve been promising, it does at least prove that it is not just me and Steven Pinker who are harping on about this!
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