HK, I suspect that those Chinese parents working in a coal mine and an apple orchard aren't highly educated. smile

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But adoption studies? The problem is that there just aren't quite enough of those subjects to make a good study group....

Actually, there have been a lot of studies in that area. For example, look at this meta-analysis from 2005. It summarized 62 studies. The abstract says that adoption raised IQand improved school performance relative to unadopted BIOLOGICAL sibs:

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This meta-analysis of 62 studies (N=17,767 adopted children) examined whether the cognitive
development of adopted children differed from that of (a) children who remained in institutional care or
in the birth family and (b) their current (environmental) nonadopted siblings or peers. Adopted children
scored higher on IQ tests than their nonadopted siblings or peers who stayed behind, and their school
performance was better. Adopted children did not differ from their nonadopted environmental peers or
siblings in IQ, but their school performance and language abilities lagged behind, and more adopted
children developed learning problems. Taken together, the meta-analyses document the positive impact
of adoption on the children’s cognitive development and their remarkably normal cognitive competence
but delayed school performance.