>, twin studies is not enough to convince me of the causation. Twin studies are controversial as they do not provide a random sample and have a number of inheri

>, twin studies is not enough to convince me of the causation. Twin studies are controversial as they do not provide a random sample and have a number of inherit biases. I wasn't talking about Twin studies. It is a fact that among adopted kids their intelligence correlates almost as much with their biological parents as for non adopted kids. Adopted kids is such a big group that there is no lack of data.