Adoption is the closest thing we have to licensed parenting, in most jurisdictions a potential parent is screened by some agency before being allowed to adopt. In situations where one child is adopted and a sibling remains with the biological parent we have the conditions for a natural experiment – a test of the importance of effects of nurture on cognitive ability.
A study published this year analyses 436 such cases. From the paper:
“Individual differences in cognitive ability result from a complex admixture of genetic and environmental influences. Adopted children are one way to estimate the degree of malleability of cognitive ability in response to environmental change in the context of a scientific design that can control for genetic differences among individuals. Sibling pairs in which one member is adopted away and the other reared by biological parents are a particularly powerful research design. In a large population-based sample of separated siblings from Sweden, we demonstrate that adoption into improved socioeconomic circumstances is associated with a significant advantage in IQ at age 18. We replicate the finding in a parallel sample of half-siblings.”