The gold standard is "separated twins" studies, looking at identical twins who are adopted away to different families at birth. Sadly for science but happily f

The gold standard is "separated twins" studies, looking at identical twins who are adopted away to different families at birth. Sadly for science but happily for the families involved, these are getting harder to do; the primary source of such twins used to be unmarried women who became pregnant, but between the availability of birth control and a dramatic reduction in the social stigma, far fewer children are being "hidden" by being adopted away.