THOSE who oppose gay marriage often argue that having gay parents is hard on children. This has been a hard argument to make, because there simply isn't that much data about the effects of growing up with gay parents, and what little there is—such as the 2010 study that found a 0% rate of child abuse in lesbian households—tends to undermine it. Some will believe that has changed this week with the publication of a new study by Mark Regnerus, a sociologist at the University of Texas, on "adult children of parents who have same-sex relationships." As Mr Regnerus explains, in an accompanying essay at Slate, there are significant differences between that group and those who grew up in intact biological families (ie, a mother and a father, no adoption, no divorce). "On 25 of 40 different outcomes evaluated, the children of women who've had same-sex relationships fare quite differently than those in stable, biologically-intact mom-and-pop families," he writes, "displaying numbers more comparable to those from heterosexual stepfamilies and single parents."

The study has been greeted with fierce criticism, and for good reason. Mr Regnerus's methodology stacked the deck against gay parents. There aren't that many young adults around who grew up with openly gay parents, so he drew a bigger circle. Anyone who reported that either of their parents had a same-sex relationship while they were growing up was put into the "lesbian mothers" or "gay fathers" category, regardless of whether their parents had been married or divorced, whether the kids were adopted or biological, whether the parents seemed happy or not, whether the same-sex affair was a one-time encounter or the basis of the household, and so on. (By this silly standard, the number of children growing up with a gay parent is about to skyrocket.)

As Jim Burroway explains, “If one wanted to intentionally create Lesbian Mothers and Gay Fathers groups which were least likely to look like an intact biological family, I can't imagine a better way to do so than to take the steps Regnerus has taken here.” Most of the people classified as having a gay or lesbian parent came from a broken home—which isn't really surprising, if you consider that all the respondents were born when gay marriage was illegal in the United States, and homosexuality was largely stigmatised. "If their kids, 20 to 40 years later, are struggling, does that reflect poorly on gay parents?" William Saletan rightly asks. "Or does it reflect poorly on the era of fake heterosexual marriages?"

Moreover, it's probably safe to assume that the study will be used by critics of gay marriage to corroborate their stated belief that it is bad for children. Although Mr Regnerus says that he's not blaming sexual orientation for anything, per se, he does seem sceptical of gay parents, and he isn't showing much common sense about the ways America has changed since 1975. He acknowledges, for example, that the current generation of gay and lesbian parents "may be forging more stable relationships in an era that is more accepting and supportive of gay and lesbian couples." It would be shocking if they weren't. But regardless of Mr Regnerus's conclusions, critics of gay marriage can take the top-line result—that there has, historically, been a difference between growing up with gay parents and growing up with an intact biological family—to make a political point. "That's unfortunate because it's illogical and unfair," writes John Corvino at the New Republic. "But it's especially unfortunate because it misses yet another opportunity to focus on actual child welfare."

That points to another regrettable phenomenon: it's become virtually impossible to have a public discussion about marriage, family structures, and child welfare without things going quickly off the rails. This kind of research, after all, isn't useless. Mr Saletan, after registering his concerns, finds some value in Mr Regnerus's study. "It tells us something important," he writes. "We need fewer broken homes among gays, just as we do among straights. We need to study Regnerus' sample and fix the mistakes we made 20 or 40 years ago." The findings, in other words, could be used to make a case against bigotry rather than against gay people: if the previous generation of gay parents showed more instability than their straight counterparts, it might be a reflection of the fact that it was hard to be gay in America during an era when homosexuality was broadly and cruelly stigmatised.