Adoptive Parents More Invested

Adoptive parents, apparently, rate better than biological parents. According to a study, published in the new issue of the American Sociological Review, adoptive parents give more to their kids by spending a lot more time with them and giving more of themselves. In short, most adoptive parents are better parents than biological parents.
This report made me go hmmmm… And I am torn between agreeing and starting a "but…" argument.
What do YOU think?