The end result is that even diseases that look largely genetic in twin studies are caused by an insanely complex confluence of factors, with hundreds of genes contributing to the disorder. (I was talking to a scientist a few weeks ago who said he wouldn't be surprised if a thousand different genes were involved in triggering the range of behaviors typically categorized as "schizophrenia.") But wait: it gets worse. The brain is a plastic machine, constantly altering its patterns of gene expression in response to environmental changes. As a result, the static texts of Nature are constantly being modified by Nurture.
Razib has more.