[shalmanese]

By studying a) identical twins who are raised together, b) fraternal twins who are raised together, c) identical twins who were adopted and raised apart at birth and d) fraternal twins who were raised apart, it's possible to tease out the effects of nature and nurture.

For example, you're far more likely to be schizophrenic if your identical twin is schizophrenic compared to your fraternal twin. This strongly suggests that schizophrenia has a major heritable component as no other explanation would account for this. This is as much "hard science" as anything else we study.