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.
I'll explain my thinking here, if you like, since I think it relates to the concepts under discussion.
The thing I'd caution, though, is that if one
believes that environment is the ONLY limiting factor in cognitive development, then any evidence that is suggestive of another causative mechanism is likely to be rationalized away.
The problem that I see with twin studies in their attempts to tease apart nature versus nurture is the flip side of the economic disadvantage coin. That is, adoptive homes are probably
more like one ANOTHER than they are as different as two homes from the full range of the socio-economic spectrum.
Therefore, the argument that adoption studies of siblings or twins can represent the influence of "environment" is highly suspect to start with, in my estimation. It's a sampling problem. Adoptive homes cannot POSSIBLY represent the various extremes of environment that exist in biological childrens' homes-- or the adoptions wouldn't have ever been approved in the first place, at the low 'enrichment' end of things. Does that make sense?
So if a 'terrible' environment is a zero and one which is the stuff of fantasy is a ten, then most adoptive homes are probably between a 4 and an 8. For the same reasons that I'd speculate that children born of fertility treatments enjoy an environmental advantage-- because adoption or fertility treatment both suggest a particular environment. One that is advantageous to children-- that is, those are going to be home environments where children are: a) treasured, b) prepared for in every imaginable way, and c) relatively affluent (since adoption of "normal" newborns or fertility treatment is pretty expensive).
Perhaps
more useful and objective would be data from homes in which some children are biological and others adopted (provided that one knew the biological parents' IQs)-- are there meaningful differences in cognition between the two categories of children in that kind of home? I'm not aware of any studies on the subject, but they may well exist.
