From the NYT Mag article quoted in extenso by Brad DeLong:

A later study of French youngsters adopted between the ages of 4 and 6 shows the continuing interplay of nature and nurture. Those children had little going for them. Their I.Q.s averaged 77, putting them near retardation. Most were abused or neglected as infants.... Nine years later, they retook the I.Q. tests.... The amount they improved was directly related to the adopting family's status. Children adopted by farmers and laborers had average I.Q. scores of 85.5; those placed with middle-class families had average scores of 92. The average I.Q. scores of youngsters placed in well-to-do homes climbed more than 20 points, to 98.... Taken together, these studies show that the issue has changed: it is no longer a matter of whether the environment matters but when and how it matters. And poverty, quite clearly, is an important part of the answer.
(bolding mine)