Identical twins studies are often flawed, because the environment difference between twins is far from random for obvious reasons. For example, in one of the (o

Identical twins studies are often flawed, because the environment difference between twins is far from random for obvious reasons. For example, in one of the (often used) minesota studies about IQ and twins, the average IQ of the father was 120, whereas the state average was 105 (see http://psycnet.apa.org/journals/bul/125/4/392/ http://psycnet.apa.org/journals/bul/125/4/392/). And while it has become a cliche, it is still true that correlation is not causation: coming up with correlation is easy, but solid scientific works need to prove that the correlation is not spurious. Most studies don't (this is sadly not specific to discussions about IQ, and is a plague of many studies in too many areas - good statistics are hard).