The best twin studies look at pairs of identical twins who were separated at birth and raised in different households. If they still end up more similar to each

The best twin studies look at pairs of identical twins who were separated at birth and raised in different households. If they still end up more similar to each other than to their adopted siblings on some trait, we conclude that trait has a significant genetic component, and more sophisticated analysis can measure how significant it is. Note that the technique I described is not strictly able to determine whether the effect is from genetics or some other pre-birth shared environment, like maternal alcohol consumption. For many practical purposes, that distinction doesn't really matter.