The following bit from Steven Levitt's Freakonomics was quite surprising to me:

There is a strong correlation—a negative one—between adoption and school test scores. Why? Studies have shown that a child's academic abilities are far more influenced by the IQs of his biological parents than the IQs of his adoptive parents, and mothers who give up their children for adoption tend to have significantly lower IQs than the people who are doing the adopting. There is another explanation for low-achieving adoptees which, though it may seem distasteful, jibes with the basic economic theory of self-interest: a woman who knows she will put her baby up for adoption may not take the same prenatal care as a woman who is keeping her baby. (Consider—at the risk of furthering the distasteful thinking—how you treat a car you own versus a car you are renting for the weekend.)...

In a paper titled "The Nature and Nurture of Economic Outcomes," the economist Bruce Sacerdote addressed the nature-nurture debate by taking a long-term quantitative look at the effects of parenting. He used three adoption studies, two American and one British, each of them containing in-depth data about the adopted children, their adoptive parents, and their biological parents. Sacerdote found that parents who adopt children are typically smarter, better educated, and more highly paid than the baby's biological parents. But the adoptive parents' advantages had little bearing on the child's school performances. As also seen in the ECLS data, adopted children test relatively poorly in school; any influence the adoptive parents might exert is seemingly outweighed by the force of genetics. But, Sacerdote found, the parents were not powerless forever. By the time the adopted children became adults, they had veered sharply from the destiny that IQ alone might have predicted. Compared to similar children who were not put up for adoption, the adoptees were far more likely to attend college, to have a well-paid job, and to wait until they were out of their teens before getting married. It was the influence of the adoptive parents, Sacerdote concluded, that made the difference.