[ronaldx]

> twins reared separately

This is the gold-standard for testing genetic association, yes, but it is increasingly considered unethical. And is not perfect - twins are still likely to be placed in similar homes (typically both in foster homes, after all), which likely results in an overestimate of the genetic effect.

Modern statistics is about handling the data that is available: present-day researchers cannot rely on a statistically perfect study which would have to be generational and eugenic in scope.