Last week, the U.S. Supreme Court heard oral arguments in two cases challenging legal restrictions on same-sex marriage. Proponents and opponents sought to cudgel one another with sociological and psychological studies aiming to prove that science is on their side. Well, what does the science say?

Impact on Traditional Marriage

Some opponents told the court that same-sex marriage will undermine conventional marriage among heterosexuals. So what do the data say about how legalizing gay marriages affects conventional marriages?

A 2009 study by University of Sherbrooke economist Mircea Trandafir investigated the effect of the legalization of same-sex marriage in the Netherlands, the first country to recognize same-sex marriage. In 1998, the Dutch created registered partnerships, which are open to all couples, and in 2001 a law allowing full same-sex marriages. His analysis found that same-sex marriage leads to a decline in the different-sex marriage rate, but not in the different-sex union (marriage plus registered partnership) rate. In other words, Dutch heterosexual couples are taking advantage of the "marriage lite" registered partnership alternative.

At the time of Prof. Trandafir's study, the chief difference between registered partnerships and marriage was that the former could be dissolved at the civil registry by mutual agreement. In a 2012 West Virginia Law Review article, Mercer School of Law professor Scott Titshaw shows that the political compromises provoked by the initial refusals to extend full marriage rights to same-sex couples result in a proliferation of civil union alternatives. Prof. Titshaw agrees with Prof. Trandafir that different-sex couples increasingly find the new marriage alternatives attractive; in effect, refusing to give full legal recognition to same-sex couples ends up diminishing the status and benefits associated with conventional marriage for everyone. Ironically, conservatives by opposing the extension of full marriage rights to gay people have ended up weakening the institution they sought to protect.

The Divorce Rate

Sweden legalized same-sex civil unions in 1995 and gay marriage in 2009. A 2011 demographic study from researchers at the University of Stockholm reports that since 1999, after decades of falling, both the marriage rate and the fertility rate have trended upward and the divorce rate is down.

Massachusetts was the first state to legalize same-sex marriage in 2004. In 2003, the divorce rate in Massachusetts was 2.5 per 1,000 residents, and it fell to 1.9 by 2009. The Massachusetts marriage rate jumped 15 percent in 2004, as many same-sex couples chose to get married, but since has remained stable. Interestingly, the states that permit same-sex marriage tend to have lower divorce rates than those that ban same-sex marriage.

A 2004 study of registered partnerships in Sweden reported that gay male couples were 50 percent more likely to divorce than were heterosexual couples. Lesbian couples were nearly three times more likely to divorce than were heterosexual couples.

But how salient are higher divorce rates among gays and lesbians for making public policy? Consider that a 2008 study in the journal Family Relations by Rice University sociologist Jenifer Bratter found that in the U.S. black-husband/white-wife marriages were twice as likely to end in divorce as white/white couples, and Asian-husband/white-wife couples were 59percent more likely. Yet few would argue that interracial marriages should be prohibited because their children are at substantially greater risk of experiencing the social, psychological, and economic disadvantages stemming from a higher interracial divorce rate.

Having Kids

Nearly 20 percent of same-sex households—i.e., 115,000—reported having children, and 84 percent contained children biologically related to one of the householders. In comparison, 94 percent of different-sex married couple households with children reported living with their own children. A study issued in February by the Williams Institute, a gay public-policy think tank at the law school of the University of California, Los Angeles, reports that 37 percent of "lesbian, gay, bisexual and transgender" (LGBT) adults have had a child at some time in their lives. In addition, the report notes that as many as six million American children and adults have an LGBT parent.

Opponents and proponents of same-sex marriage spar fiercely over the data about how children fare in same-sex households. On March 21, when the American Academy of Pediatricians issued a statement in favor of same-sex civil marriage, the group also published a technical report that comprehensively looked at the available research on the well-being of children living in same-sex households. The report noted that a big problem with current research is the small sample sizes of many of the studies. An additional problem is that most of the children in these studies have been through divorce before living in a same-sex household. Divorce is well known to have deleterious effects on the well-being of children.

However, data are reassuring from the National Longitudinal Lesbian Family Study, which includes 78 lesbian families who used donor sperm to have children and have been followed since the 1980s. A 2012 study compared quality-of-life measurements of adolescents from lesbian families with those from a matched set of adolescents raised in different-sex homes. The researchers reported that "adolescents reared by lesbian mothers from birth do not manifest more adjustment difficulties (e.g., depression, anxiety, and disruptive behaviors) than those reared by heterosexual parents."

By the time that their children were age 17, some 55 percent of the lesbian couples had separated compared to 36 percent of heterosexual couples in the National Survey of Family Growth. However, children from separated lesbian couples don't appear to manifest the social and psychological problems often found among children whose heterosexual parents are divorced. The better scores achieved by the children of lesbians, the researchers point out might result from the fact that 75 percent of the separated lesbian couples shared custody, whereas 65 percent of divorced heterosexual mothers had sole custody of their children.

A 2010 study in the periodical Demography by Stanford University sociologist Michael Rosenfeld parsed Census data to compare the school progress of children reared in same-sex, heterosexual, and single parent families. He reported that "children raised by same-sex couples have no fundamental deficits in making normal progress through school."

Adoption Outcomes

A 2012 study by UCLA researchers involving 82 families (60 heterosexual, 15 gay, and 7 lesbian) who adopted high-risk children from foster care found that on average, children in both same-sex and different-sex households "showed significant gains of approximately 10 IQ points in their cognitive development and maintained stable levels of behavior problems that were not clinically significant."

The researchers noted that these findings were especially remarkable because the children adopted by same-sex couples were generally higher risk and often of a different ethnicity than those adopted by heterosexual couples. The bottom line is that research on the effects of being reared by same-sex parents on children is certainly not perfect, but the AAP seems right when it concluded that despite research "imperfections, it is likely that the extensive research efforts that have been carried out would have documented serious and significant damages if they existed."

Monogamy

Research suggests one salient difference between same-sex, especially gay male couples, and different-sex couples relates to the acceptability of sex with people outside of the relationship. A 2010 study by University of Toronto sociologist Adam Isaiah Green in the Canadian Journal of Sociology involving 30 same-sex married couples around Toronto found that two-thirds of same-sex spouses (40 percent female, 60 percent male) did not believe marriage needed always to be monogamous. In fact, nearly half of male same-sex spouses (47 percent) had an explicit agreement that allowed for non-monogamy. In comparison, the General Social Survey reported in 2010 that 19 percent of men and 14 percent of women they had been unfaithful at some point during their marriages.

My reading of the scientific literature as it currently stands is that the legalization of same sex marriage does not have major effects on marriage trends in the wider society. As ever greater numbers of gay, lesbian, bisexual, and transgender Americans have exited the closet, more straight Americans have come to know and accept their homosexual family members, friends, and colleagues. It is this personal data, not the dueling studies published in obscure social science journals, that have now persuaded a majority of Americans in recent polls to support same sex marriage.

A slightly different version of this article originally appeared at the Wall Street Journal's Ideas Market.

Disclosure: My wife and I have supported Equality Virginia for a number of years.