Few adoptive parents are going to be in the socioeconomically disadvantaged category and have an adopted twin for a study, so I'm guessing that the relative differences between adoptive homes show only the limits of just how much heredity can contribute
I admit to being baffled by this statement, even though IANASANPTBO. (Requests by PM to explain this standard acronym will be satisfied with alacrity.) I would think that if adoptive homes did tend to be similar, it still wouldn't show the upper limit of how much heredity can contribute because that's in tension with the unknown quantity of how much the environment can contribute. I guess I don't buy the idea that environment matters only up to a sufficiency level at or below that of the average adoptive home, or, if you like, that the average adoptive home is ideal.
