Large-sample social science examining family structure finds that children raised by their biological parents, on average, fare better. That’s one reason state policies promoting traditional marriage are both rational and beneficial.

When a man and a woman marry and have children, that union has a strong impact on those children and, consequently, a strong impact on the society of which those children become a part. For decades, social science researchers have investigated how family structure affects children by measuring their social and academic progress.

This social science data-gathering dispassionately can help society discern what type of family structure, on average, helps children the most. It can help to guide rationally what policies encourage and promote the kind of stable family life that leads to human flourishing.

“Research clearly demonstrates that family structure matters for children, and the family structure that helps children the most is a family structure headed by two biological parents in a low-conflict marriage,” wrote Kristen Anderson Moore, Susan Jekielek and Carol Emig in a research summary for ChildTrends. “(I)t is not simply the presence of two parents, as some have assumed, but the presence of two biological parents that seems to support children’s development.”

There is widespread agreement about this fact when looking at traditional marriage. Now, as courts, legislators and social scientists turn to the hotly contested issue of same-sex marriage, should this wealth of existing evidence about family structure be ignored? No. In fact, family structure research is more relevant than ever before.

This week, the 10th Circuit Court of Appeals in Denver will hear an appeal of the federal district court decision that temporarily struck down Utah’s constitutional amendment defining marriage as between a man and a woman. Other state constitutions are also being litigated. A challenge to Oklahoma’s traditional definition of marriage will be heard soon by the same three-judge appellate panel.

Emotions about this issue are understandably high. Indeed, emotion is at the heart of the effort to redefine marriage from a conjugal relationship of responsibility for the rearing of children to a romantic attachment for adults. But as our federal judicial system considers a wholesale change to how the foundational institution of marriage is defined within our federal constitutional system, reason needs to hold sway.

We applaud the vigorous yet respectful defense of traditional marriage mounted by Utah Gov. Gary Herbert and Attorney General Sean Reyes. In addition to the constitutional arguments for reversing the district court decision, Utah citizens and lawmakers have many sound and rational policy reasons to retain the traditional definition of marriage.

Utah highlighted these reasons by noting social science evidence throughout its opening brief, focusing on the impact of family structure from dozens of studies, journal articles and books. The brief showed how “the two sexes bring different talents to the parenting enterprise,” how “the weight of scientific evidence seems clearly to support the view that fathers matter,” how the absence of a father places a daughter at special risk for early sexual activity, that children in same-sex households experience lower high school graduation rates, and that there are higher rates of depression, delinquency and substance abuse among children conceived through sperm donation.

Some advocates of same-sex marriage dismiss or attempt to discredit this research. In the Utah case, the plaintiffs largely ignored the state’s evidence. In same-sex battles in other states, including Michigan, critics engaged in ad hominem attacks, attempting to rebut research by pointing out how many sociology professors at the researcher’s university disagree with the implications of his study.

Some point to the American Psychological Association’s brief on lesbian and gay parenting, which looked at 59 studies on the subject and concluded that “not a single study has found children of lesbian or gay parents to be disadvantaged in any significant respect relative to children of heterosexual parents.”

But Louisiana State University professor Loren Marks examined these 59 studies in excruciating detail and found that each of them relied upon small, self-selected samples of lesbian or gay parents. The aggregation of these convenience-based samples is less statistically significant than results from robust comprehensive data sets now in use by researchers in the U.S. and Canada.

Yet these more comprehensive studies are being held to a double standard. To those who critique the lack of stable same-sex couples in such data sets, the proper response is that it is not a “fatal flaw” to examine facts from a population-based data set.

“Children in homes with their married biological parents perform better than the other family types measured; that is, compared to cohabitating, single, or same sex households,” said Douglas Allen, an economics professor at Simon Fraser University who has studied results from both the 2000 U.S. Census and the 2006 Canadian Census.

In this last study, Allen found that girls in a home with homosexual fathers had a 15 percent chance of graduating, compared with girls in a home with a mother and a father. Girls in a home with lesbian mothers had a 45 percent chance of graduating, relative to the mother and father. The relative graduation ratio for girls with a single dad was 46 percent, and 51 percent for girls with a single moms. The point of such research is not about casting aspersions on any particular parents, straight or gay, but about understanding the consequence — on average — for a child raised in such a union.

As the 10th Circuit reviews the creative constitutional interpretation of Utah District Judge Robert Shelby, one of the standards it will consider is whether Utah’s decision to define marriage as between one man and one woman meets a so-called “rational basis test.” Large-sample social science examining family structure find that children raised by their biological parents, on average, fare better. That’s one reason why state policies promoting traditional marriage are both rational and beneficial.

As the Deseret News editorialized in October 2013, “It makes no sense for society to fundamentally alter such an institution without understanding the consequences.”