When you look for stuff which correlates with the outcome of a child growing up, almost nothing correlates <i>except</i> parenting quality. For example, single parenting and divorced parents correlate with lower outcomes [1]. Parental class or wealth do not correlate except in regards to how people of different classes or wealth situations raise children differently, or have more discretionary time available to spend on their children.<p>This paradox flies in the face of the conventional interpretation of the data. This really calls for follow-up studies which look more carefully for causes -- for example, as vessenes points out, children who are available for adoption may have, on average, challenges that differ from the norm.<p>[1] I am not judging. Instead, consider that a single programmer will generally, but not always, be less effective than a pair of programmers.