Since I’ve written about this topic at some length in the past, a quick note on John McCain’s recent statement about gay adoption:

I think that we’ve proven that both parents are important in the success of a family so, no I don’t believe in gay adoption.

Now, possibly there’s new or better data since last I looked at this, but my understanding is that this isn’t quite true. What we’ve proven is that both biological parents are important in the success of the family. That is, kids raised by their biological mother and father do, indeed, tend to do better on a whole host of measures than kids raised by single parents or in step-parent families. But especially if you control for the advantages of a second income, adding a second non-biological parent actually delivers pretty negligible benefits, on average, over the single-parent household.

In other words, there’s very good evidence supporting the general proposition that, other things equal, the best family structure is two biological parents. By definition, this is not what you have in cases of adoption. There’s some reason to think it’s better to have a second (non-biological) caretaker than not, but really much less than you might expect. But if we’re not talking about both biological parents, then as far as I’m aware, there is actually no good evidence at all supporting the idea that having one mommy and one daddy, rather than two moms or two dads, gives you any real benefit.