Yes that is valid point about the limitation of this approach. There are 3 methods to address it, none of them are perfect: (1) Statistically compare the popul

Yes that is valid point about the limitation of this approach. There are 3 methods to address it, none of them are perfect: (1) Statistically compare the population of twins vs the population of non-twins. The paper I linked to has more specifics on this, but twins earn similar amounts of money, live similar lengths of time, do similarly well in exams vs. non-twins, so it any twin-specific genetic effect has to simultaneously be strong enough to cause this discrepancy, but not strong enough to impact population means. (2) Compare adoptees to genetic offspring of parents. This has the same issue that you pointed out (maybe there is something specific about adoptees) but the two together find roughly similar effect magnitudes, which reenforce each-other. I'm actually a bit more concerned about adoption being less 'clean' an identification strategy than twin studies, because it's a complicated social process. (3) Look at genetic variation at population level and find genetic markers that explain variation, using Mendelian randomization as identification strategy - this is technically very challenging, and has only recently become feasible. So far research have only been able to explain around 7% of IQ for example with specific genetic markers, but it will be interesting to see if this creeps up as the field matures. The big puzzle here is the large variation that we between cultures - moving time-periods, or countries can significantly change your life-expectancy, earnings, test-scores or even IQ. I don't know how to reconcile this with the strange finding of this research strand is that family level environmental effects (shared environment) seem to be much less important than the conventional wisdom would suggest. Links https://www.sciencedirect.com/science/article/pii/B0080427073001759 https://www.sciencedirect.com/science/article/pii/B008042707... (gated) https://en.wikipedia.org/wiki/Mendelian_randomization https://en.wikipedia.org/wiki/Mendelian_randomization https://www.newscientist.com/article/2163484-found-more-than-500-genes-that-are-linked-to-intelligence/ https://www.newscientist.com/article/2163484-found-more-than... https://en.wikipedia.org/wiki/Flynn_effect https://en.wikipedia.org/wiki/Flynn_effect