> > In fact, you can correct for the exact problem you describe with sibling studies where people have the same environment but different genes. > You need to

> > In fact, you can correct for the exact problem you describe with sibling studies where people have the same environment but different genes. > You need to choose a study design that's correct in the first place. That's precisely what a sibling study is: a study design that's correct in the first place, as far as the problem we're discussing goes. > You're describing a natural experiment That is correct: a sibling study, which corrects for the problem we're discussing, often contains aspects of a natural experiment, namely that the siblings tend to naturally be siblings, rather than raised together purely for the purpose of a study :)