From the article:
Okay, but perhaps they were smarter and had other important attributes, like an understanding of what it takes to raise a successful child. Poor people who give up their children for adoption probably possess certain characteristics that poor people who keep their children do not. Maybe that adds to the adopted child’s future success.
As an aside, a friend who lived in Sweden for 20 years told me there were no adopted children there, since the liberal view of abortion pretty much assured that no unwanted children were born. That may have been a generalization.