I believe that there is one small French adoption study which produced 1 sigma IQ difference between the bottom and top buckets (deciles?) of adopting families.

• Christiane Capron & Michel Duyme (1989), "Assessment of effects of socio-economic status on IQ in a full cross-fostering study", Nature, 340, 552-554
• Christiane Capron & Michel Duyme (1996), "Effect of Socioeconomic Status of Biological and Adoptive Parents on WISC-R Subtest Scores of their French Adopted Children", Intelligence, 22, 259-275

(Both references describe the same study.) Capron & Duyme found 38 French children placed for adoption before age 2, 20 of them to parents with very high socioeconomic status (operationalized as having 14-23 years of education and working a profession) and 18 to parents with very low socioeconomic status (unskilled & semi-skilled labourers or farmers, with 5-8 years of education). When the kids took the WISC-R IQ test, those adopted into the high-SES families had a mean IQ of 111.6, while those in the low-SES families had a mean IQ of 100.0, for a difference of 0.77 sigma.