See related posts SES and IQ and Random microworlds.
Note to Tiger Moms and Sociologists: Shared genes make people more alike, but shared family environment does not (very much). Feel free to disregard, though. Who needs data when you have an opinion? :-)
Average age at adoption was 4.7 months. I don't know why the IQ means are all above average. (See comments; the researchers might have used an old norming of the WISC/WAIS.) The study was done in Minnesota: the adopted children were 67% Asian (avg 107.7) and 20% Caucasian (avg 105.2) while the families were 95% Caucasian.
The Environments of Adopted and Non-adopted Youth: Evidence on Range Restriction From the Sibling Interaction and Behavior Study (SIBS)
Behav Genet (2007) 37:449–462
Abstract: Previous reviews of the literature have suggested that shared environmental effects may be underestimated in adoption studies because adopted individuals are exposed to a restricted range of family environments. A sample of 409 adoptive and 208 nonadoptive families from the Sibling Interaction and Behavior Study (SIBS) was used to identify the environmental dimensions on which adoptive families show greatest restriction and to determine the effect of this restriction on estimates of the adoptive sibling correlation. Relative to non-adoptive families, adoptive families experienced a 41% reduction of variance in parent disinhibitory psychopathology and an 18% reduction of variance in socioeconomic status (SES). There was limited evidence for range restriction in exposure to bad peer models, parent depression, or family climate. However, restriction in range in parent disinhibitory psychopathology and family SES had no effect on adoptive-sibling correlations for delinquency, drug use, and IQ. These data support the use of adoption studies to obtain direct estimates of the importance of shared environmental effects on psychological development.