If you’ve paid much attention to the debate over gay adoption—and even if you haven’t—you’ve probably head some pundit or pol assert that “children need a mother and a father,” a claim that’s usually linked to the notion of gender complementarity, the idea that mom and dad each bring something unique to the parenting table, such that a child who doesn’t get both mom-parenting and dad-parenting misses out, presumably with dire consequences. They point out that the many, many studies showing homosexuals are fit parents were mostly comparisons of heterosexual and lesbian single mothers, a comparison (so the argument runs) that hides these important complementarity benefits.

Except here’s something I’ve noticed in the course of researching another marriage-related piece I’m working on. When conservatives make the case for the importance of stable marriages but don’t have gays on the brain so much—especially when they’re talking about the harms of divorce—they tend to allude to the social science consensus that what’s really benificial for kids is to be raised by both (married) biological parents. In other words, it’s true that kids raised by single moms don’t fare as well as kids raised by both biological parents, but kids raised by a heterosexual parent who’s remarried don’t actually do any better once you control for the economic benefit of having a second income in the house. That suggests that it’s that biological link, rather than the mere presence of one parent of each gender, that’s key to the better outcomes.

But what this means is that all the marriage literature conservatives like to cite as an argument against gay parenting is basically useless in the context of adoption. An adopted child, pretty much by definition, is not going to be raised by both biological parents. But in the absence of that biological link, the studies seem to suggest, the only reason to prefer having a kid raised by a couple is the economic benefit—and a couple of gay men can provide that as easily as a hetero couple. Oddly, this crucial point, at the forefront of conservative arguments when divorce is at issue, seems mysteriously absent when it comes to the argument about gay adoption.