In response to Why Am I Right-Handed? | FiveThirtyEight : I have been aware of the strength of genetic correlations to intellectual and personality traits for over 20 years, and I get irritated by people that more recently learning of such things tending to take an "it's all the genes" mentality. One evening, after being annoyed yet again, this time by someone overgeneralizing about mental health, I went looking for such studies, but found something interesting, the placental environment. Studies of twins show the correlation with mental health issues to be strong, but one study attempted to asses the likelihood of having a shared or separate neonatal environment. The result was that if shared, the correlation shot up to ~.8, but if separate it dropped to about .3. It adds an interesting nuance to the genes/environment debate.