A new study from the University of Colorado Denver finds that there has scientific consensus on same-sex parenting for decades.

By assessing a number if studies that examined same-sex parenting, and studying the trends and shifts between them, the researchers were able to determine that the scientific community agrees there were no differences in children raised by same-sex couples and different-sex couples.

Despite arguments made to the contrary just this year in Supreme Court amicus briefs, the consensus is not new.

According to the study, there was already a developing consensus affirming same-sex parenting among social scientists by 1990. By 2000 and henceforth, that consensus has been “overwhelming.”

Lead researcher Jimi Adams, associate professor in the Department of Health and Behavioural Studies at CU Denver College of Liberal Arts and Sciences, told Think Progress that even though there is still the occasional dissent…

… even those dissenters seem to agree that consensus exists. They’re forced instead to claim that they think that the existing consensus is pre-mature.”

Adams, along with co-author Ryan Light of the University of Oregon, found that studies conducted before 1990 “disproportionately focused on same-sex parenthood that occurred following dissolved heterosexual partnerships.”

These studies thus couldn’t isolate the effect of having same-sex parents on children from the effects of divorce and separation. As researchers began to focus on same-sex couples who adopted or used in vitro fertilisation — thus raising their children from birth in a stable household — the differences disappeared and consensus began to lock in.

Even when differences are still found among children of same-sex couples, it’s not necessarily because of the sexual orientation of the parents.

As Adams explained.

.. [relationships] have had a tendency to be more unstable [and] parental relationship instability is associated with negative outcomes for kids.”

But cultural stigma and legal inequality could contribute to that instability, so the progress of marriage equality could likely “lead to beneficial changes for these other sources of potential negative impacts on kids.”

It’s these other factors that contemporary dissenters like Mark Regnerus and Loren Marks fail to account for. Regnerus’ study, which purported that the children of same-sex couples fare worse, conflated children whose parents separated and then entered a same-sex relationship with those raise by same-sex couples from birth. As Adams and Light point out, a recent re-analysis of Regnerus’ data controlling for these factors actually supports the “no differences” consensus.

Marks’ dissent does not stem from new research he conducted, but from a critique of past studies because they use small convenience samples — methods of recruiting participants that aren’t totally random, such as snowball samples, where one same-sex family might help recruit another that they know — which he believes bias the results.

Adams told ThinkProgress that “it’s really difficult to gain much leverage on the observable patterns from large population-based samples like Marks/Regnerus are claiming we need more of” because those random samples simply don’t capture enough kids raised by same-sex couples.

Likewise, Adams isn’t persuaded by Marks’ critiques of convenience samples.

Any one convenience sample can rightfully be criticised regarding its lack of ability to generalise its findings to the broader population from which it was drawn.

But when study after study finds the same thing — each with their own separate means for drawing their convenience (or small scale) samples — those weaknesses become less and less likely to be able to account for the mounting consistency of the resulting evidence.”

Adams admits that sometimes a consensus can be re-evaluated, and there is certainly still new information to be collected on the matter of same-sex couples.

Perhaps the most glaring gap is how few of the kids in these studies are from married same-sex homes.”

As marriage equality expands, there will be more opportunities for “apples-to-apples” comparisons, but he expects such studies would only confirm the consensus, if not reveal some new advantages that the children of same-sex couples experience when their parents can marry.

The cumulation of evidence we have to this point — according to our analyses — appears to be pretty robust. So, I think it would take some earth-shattering new evidence to upset this applecart.”