During oral arguments in Hollingsworth v. Perry—the Supreme Court case involving California’s Proposition 8—Justice Kennedy asked a very important question: In its potential impact on children and society, wouldn’t imposing same-sex marriage on unwilling states be akin to “jumping off a cliff” and subjecting the nation and its children to whatever unseen dangers might lurk at the bottom?

According to a group of 100 academic marriage scholars, Justice Kennedy was right to be concerned about the harmful social effects of such a redefinition of marriage—especially on the children of heterosexuals. In fact, according to an amicus brief recently submitted in the pending Supreme Court marriage case that I filed on behalf of those scholars, the results of such a ruling could well be catastrophic. As the brief demonstrates, based on data from nations and US states that have adopted same-sex marriage, it is reasonable to predict that, over a generation, a forced redefinition of marriage would produce at least a 5 percent reduction in heterosexual marriage rates. That would result in an increase of nearly 1.3 million never-married women, and an increase of nearly 600,000 functionally fatherless children.

But why would redefining marriage reduce heterosexual marriage rates? Is it really plausible that the marriage of a lesbian couple might cause a heterosexual young adult next door to forgo marriage altogether? According to the marriage scholars’ brief, mandatory same-sex marriage would create a substantial risk of reduced heterosexual marriage rates—not because of individual same-sex marriages, but because the institutionalization of same-sex marriage necessarily requires replacing the gendered “man-woman” definition with a genderless “any qualified persons” definition. And that change from a gendered to a genderless understanding would undermine some of the key, secular norms that, among other things, encourage heterosexuals to marry.

The Man-Woman Understanding of Marriage Benefits Society and Children

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Marriage is a complex social institution that, like all social institutions, regulates and encourages certain human behaviors. Without effective social institutions, no amount of law and law enforcement can make a society function properly. Marriage reinforces particular values and actions that benefit society, both broadly and individually. As Professor Amy Wax has observed: “Marriage’s long track record as a building block for families and a foundation for beneficial relations between the sexes suggests that ordinary people desperately need the anchor of clear expectations, and that they respond to them.” Or, as the Sixth Circuit put it, at least some citizens “may well need the government’s encouragement to create and maintain stable relationships within which children may flourish.”

That is why states have traditionally supported man-woman marriage, an institution that has historically and universally been linked to procreation, marking the boundaries where sexual reproduction is socially commended. This underlying message helps achieve a principal purpose of marriage: any children born will have a known mother and father who have the responsibility to care for them. Even ancient Greek and Roman societies understood this. Despite encouraging same-sex intimate relations, they limited marriage to man-woman unions.

Of course, marriage provides benefits to adults as well. But these are secondary to the main purpose of an institution that, in the words of revered psychologist Bronislaw Malinowski, is “primarily designed by the needs of offspring, by the dependence of the children upon their parents.” Indeed, as the religious skeptic Bertrand Russell candidly observed, “But for children, there would be no need for any institution concerned with sex.”

From this purpose—ensuring the care of any children born to man-woman unions—flow several specific secular norms, norms that are “taught” and reinforced by the man-woman definition and understanding of marriage:

1. Biological Bonding and Support: Where possible, every child has a right to be reared by and to bond with her biological father and mother. And every child has a right, whenever possible, to be supported financially by the man and woman who brought the child into the world.

2. Gender Diversity: Where possible, a child should be raised by a mother and a father who are committed to each other and to the child, even where he cannot be raised by both biological parents.

3. Postponement: Men and women should postpone procreation until they are within the committed, long-term relationship of marriage. This is alternatively called the “responsible creation” or “channeling” norm.

4. Valuing Procreation/Child-Rearing: Within the protection and stability of marriage, the creation and rearing of children are socially valuable.

5. Exclusivity: For the sake of their children, men and women should limit themselves to a single procreative partner.

All of these specific norms are grounded in and support the more general norm of child-centricity: Parents and prospective parents should give the interests of their children—present and future—equal if not higher priority than their own.

Common sense and social science show that these norms provide immense benefits to children, their parents and society. In short, children generally do best emotionally, socially, intellectually, and economically when reared in an intact home by both biological parents. More specifically, as the brief documents in detail, compared to any other family structure, children raised by their biological, married parents are less likely to commit crimes, experience teen pregnancy, have multiple abortions, engage in substance abuse, suffer from mental illness, or do poorly in school. They are also more likely to support themselves and their own children in the future. No other parenting arrangement comes close (on average) to that of a child’s biological, married mother and father.

This is true because of the power of the norms stemming from man-woman marriage. For instance, biological bonds between parents and their child deepen their investments in their relationships with each other and with the child. Further, having both a mother and a father provides crucial gender diversity for a child’s social and emotional development. As famed anthropologist (and atheist) Margaret Mead noted: “One of the most important learnings for every human child is how to be a full member of its own sex and at the same time fully relate to the opposite sex. This is not an easy learning; it requires the continuing presence of a father and a mother.”

Vibrant child-centricity and biological support norms lead to less physical and sexual abuse, neglect, and divorce. And parents who embrace the procreative exclusivity norm are unlikely to have children with multiple partners—a phenomenon that leads to social, emotional and financial difficulties for children and their mothers. Similarly, people who embrace the postponement norm are less likely to have children without a second, committed parent—another well-established predictor of psychological, emotional and financial heartache.

On the other hand, a culture that largely rejects the social value of creating and rearing children jeopardizes a society’s ability to reproduce itself. It is thus not surprising that some courts have deemed man-woman marriage “the fundamental unit of the political order … [for] the very survival of the political order depends upon the procreative potential embodied in traditional marriage.”

Removing the Man-Woman Definition Would Create Enormous Risks

Since the law can alter social institutions and therefore alter social norms, removing the man-woman definition of marriage creates enormous risks to society, especially to the children of opposite-sex couples.

Even proponents of same-sex marriage admit the impact would be profound. Yale Law Professor William Eskridge, a long-time advocate of same-sex marriage, conceded that “enlarging the concept to embrace same-sex couples would necessarily transform [the institution of marriage] into something new.”  And in an article titled “Retying the Knot,” same-sex marriage architect E.J. Graff exulted that

Same-sex marriage is a breathtakingly subversive idea . . . [If it ever] becomes legal, [the] venerable institution [of marriage] will ever stand for sexual choice, cutting the link between sex and diapers.

By cutting the link between sex and children that is embedded in the traditional man-woman definition, such a redefinition would undermine each of the associated secular norms of man-woman marriage, to the detriment of the nation’s children.

For example, by embracing a vision of marriage in which the gender of parents is irrelevant, the state sends the message that a child does not need or deserve both a father and a mother. This destroys the gender-diversity norm, and saps the biological bonding and support norm. Likewise, cutting the link between “sex and diapers” in marriage eviscerates the postponement and procreation norms, with fewer children born within marriage, and fewer children born overall.

This “breathtakingly subversive” metamorphosis will not affect all equally. It will especially influence heterosexual men, who generally need more encouragement to marry and have children than women. The metamorphosis of marriage from a gendered to a genderless institution conveys to men (and women) that society no longer needs men to bond to women to form well-functioning families or to raise happy, well-adjusted children. And that message is especially likely to be influential among those on the margins: the poor, the relatively uneducated or others who are highly influenced by cultural messages promoting casual or uncommitted sex.

The weakening or destruction of these norms would result in fewer marriages, more procreation out of marriage, and a higher percentage of children raised in a home without both parents—usually without a father. The consequences would be stark and disastrous: more childhood poverty; increased psychological and emotional problems; more teenage pregnancy; poorer performance in school; higher amounts of substance abuse; more youth committing crimes; and more girls undergoing abortions.

Courting the Collapse of Marriage

If all of this seems too theoretical, one must only remember the no-fault divorce revolution, and the destruction it left in its wake. Only with the benefit of hindsight can we clearly see that no-fault divorce fundamentally altered the institution of marriage, making it more adult-centric and reducing marriage rates along the way. The divorce culture, which started at the margins, has now penetrated the institution of marriage to its core. And the collapse of marriage among the poor in our inner cities reveals how susceptible the institution is to legal alteration, and how devastating its demise can be for children. Like a fragile ecosystem, the institution of marriage did not spring into being overnight, but it can be irreversibly damaged in a hurry.

Evidence from states and nations that have legalized same-sex marriage confirms the danger that redefining marriage poses to children of heterosexuals. For example, statistical analysis of experience in the Netherlands—the first nation to legalize same-sex marriage—showed, after controlling for other factors, that the nationwide annual decline in opposite-sex marriage rates for Netherlands women aged eighteen to twenty-two went from 2.8 percent to 7.8 percent—a net decline of 5 percent. When the population was further subdivided, the net decline in the marriage rate was even greater among some populations: a decline of 31.8 percent for young women in the four largest (and least religious) urban areas, and a 13.4 percent decline for young women who were born in the Netherlands.

Descriptive data from other nations and American states where sufficient multi-year data are available show similar results. Since legalizing same-sex marriage, Canada’s and Belgium’s opposite-sex marriage rates dropped 4.3 percent and 7.7 percent, respectively. Spain has witnessed its opposite-sex marriage rates plummet 36 percent since the year it simultaneously legalized same-sex marriage and liberalized its divorce laws. And in the United States, Vermont (5.1 percent), Connecticut (7.3 percent), Massachusetts (8.9 percent), and Iowa (9.2 percent) have all seen opposite-sex marriage rates decline in the wake of legalizing same-sex marriage.

Of course, correlation is not causation. But correlation in harmony with theoretical predictions, past experience, and other causal analysis (see the Netherlands study), gives cause for grave concern.

Even a 5 percent drop in marriage rates in the United States over the next thirty years would have deep and dire consequences. The logic is simple: fewer women marrying means fewer children born, more children born out-of-wedlock—and indeed, more children aborted. Conservatively estimating that only half of that 5 percent increase in the number of unmarried women would result in women who never marry, this means that there would be 1.275 million more women who never marry and almost 600,000 more children who will be born outside of marriage. This would result in as many as 1.75 million children who will never be born, including about 900,000 who will be aborted. That’s a big deal.

On the other side of ledger, same-sex marriage proponents argue that the above-mentioned benefits of marriage to children are needed for the 240,000 children currently being raised by same-sex couples in the United States. But first, that number is only one-third of one percent (0.33 percent) of the nearly 74 million children in this country. And second, it is not clear that the benefits to children from man-woman marriage will automatically transfer to the children of same-sex couples if those couples can legally marry. Man-woman marriage and same-sex marriage empirically differ in a host of ways. And the one study to look at the differential impact on children of married versus unmarried same-sex couples showed that children were actually worse off when the couples married.

In sum, man-woman marriage needs not redefinition, but reinvigoration. And all of us, especially the Supreme Court, would do well to heed the warning of G.K. Chesterton that “nobody has any business to destroy”—or even to substantially modify—“a social institution until he has really seen it as an historical institution.” Chesterton continued:

If he knows how it arose, and what purposes it was supposed to serve, he may really be able to say that they . . . are purposes which are no longer served. But if he simply stares at the thing as a senseless monstrosity that has somehow sprung up in his path, it is he and not the traditionalist who is suffering from an illusion.

Far from being a “senseless monstrosity,” the institution of man-woman marriage has endured throughout human history because it has been shown to bring enormous benefits to children. Given the risks that a redefinition would bring—especially to children of heterosexuals—that decision should be left to the States and their people, and should not be imposed upon them by an unelected Supreme Court.