The Supreme Court has agreed to hear legal challenges to state and federal laws that define marriage as the union of a man and woman. As the Court prepares to decide these cases, one concern ought to be avoiding injury to the Court’s legitimacy. If the Court rules on the basis of something other than law—even if the Court is reasonably perceived to be morally partisan—then it will undermine the public’s faith in its commitment to the rule of law, a faith already shaken by the perception that the Court has resolved questions that ought to be left to political and cultural institutions.

As many authors have forcefully argued at Public Discourse, one cannot determine how marriage ought to be defined in law without first addressing the question of what marriage is. Therefore, there is no morally neutral ground upon which to decide which relationships should be called marriages.

Nevertheless, there remains a way for the Court to resolve the narrower legal issues presented to it without foreclosing public deliberations on the more profound philosophical questions about marriage. A recent decision out of the United States District Court in Hawaii, in the case Jackson v. Abercrombie, suggests a way for the Supreme Court to move forward without (further) undermining its prestige and integrity. If the Supreme Court is looking for morally neutral ground upon which to stand, it could do no better than to read this decision carefully.

Like California, Hawaiians struck a considered compromise on the marriage question. Hawaii extends to same-sex couples who enter into civil unions all of the rights and privileges of marriage, and reserves the term “marriage” for one-man-one-woman unions. For this reason, the challenge to Hawaii’s laws, like a similar challenge arising out of Nevada and the challenge to California’s Proposition 8 that the Supreme Court is preparing to hear, threatens to lure the judicial branch onto dangerous ground. The courts cannot require these states to recognize same-sex relationships as marriages on the ground that same-sex couples are entitled to the benefits, protections, rights, and responsibilities of marriage; these states have already extended those entitlements to same-sex couples. The only remaining basis for judicial action on behalf of same-sex couples is a ruling that the intimate commitment of a same-sex couple is a marriage. That ruling would codify in the fundamental law of the land the moral beliefs of judges.

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Wary of that danger, the district court judge in the Abercrombie case reviewed Hawaii’s laws under a rational basis standard. Rational basis review, the court explained, is the “paradigm of judicial restraint.” It prevents a court from acting as a “superlegislature to judge the wisdom or desirability of legislative policy determinations.” Avoiding the slough of moral disagreement, the court confined itself to the narrow legal question of whether the claimants, who wanted the court to invalidate Hawaii’s marriage and civil union arrangement, had negated all of the conceivable rational bases for Hawaii’s laws.

The court identified three grounds upon which Hawaii lawmakers could reasonably distinguish between marriages and civil unions. First, “the legislature could rationally conclude that defining marriage as a union between a man and woman provides an inducement for opposite-sex couples to marry, thereby decreasing the percentage of children accidently [sic] conceived outside of a stable, long-term relationship.” Second, the court found it reasonable to conclude that, where possible, “it is best for children to be raised by a parent of each sex.” Third, the court determined that “the state could rationally conclude that it is addressing a divisive social issue with caution.”

The third basis is particularly interesting. The Hawaii legislature had first created “reciprocal-beneficiary relationships,” then “passed a civil unions law, conferring all of the state legal rights and benefits of marriage (except the title marriage) on same-sex couples who enter into a civil union.” The court ruled that Hawaii must be allowed to observe the effects of these changes before deciding whether to change the institution of marriage itself. Judicial action would “short-circuit” this process. By making accommodations for same-sex couples while reserving the word marriage for conjugal unions, the state advanced its interest in cautiously experimenting with new relational arrangements while preserving the “societal understanding of the link between marriage, procreation, and family structure.”

This third rational basis provides an independent reason for states to distinguish between marriages and civil unions. And here lies a path to safety for the Supreme Court.

Ignore the question of what marriage is. Consider instead the prudential question of how states can best go about harnessing human sociability to serve public ends.

In order to answer this question, one must differentiate among different forms of human sociability. Different relationships display different characteristics and produce different social benefits. If the state is to promote those social benefits, it must be able to identify them with the relationships that produce them. The ability of state lawmakers to differentiate among different types of relationships is therefore an independent rational basis for allowing states to classify different relationships using different terms. Different institutions can correlate with and serve different types of sociability, and can incentivize the most useful aspects of each type of relationship.

State legislatures might rationally separate institutions not hierarchically along one axis but in a matrix based upon a number of prudential factors. The possibilities are nearly endless. For example, governments might notice that adult offspring caring for elderly parents provide a useful service, relieving the state of financial obligations that it would otherwise incur. They could, based upon these findings, rationally create a new legal institution to assist the efforts of the caregivers.

This is nothing new; matrices already exist in the law of every state. Governments draw legal boundaries and protections around religious organizations, trade unions, schools, businesses, and other types of sociability that the state did not create, but which it recognizes and harnesses for public ends. Marriage and the civil union are two such institutions.

Not every distinction between types of sociability carries moral stigma. Any type of sociability that the state identifies in law will necessarily exclude someone. The question is not whether the state should be permitted to exclude any group of people from a legal category. The question is whether any exclusion is justified by a rational state interest in being able to distinguish one group from another.

Vindicating moral claims to inclusion is treacherous business for courts. Same-sex couples are not the only citizens who have moral interests in how the government defines marriage. For example, the adult daughter caring for her ailing mother has been left out of both marriage and the civil union. Any moral claim that a same-sex couple has to be included in marriage extends also to the daughter and mother. The daughter is exercising her personal autonomy to bring about a desirable state of affairs in her personal relationship with her mother. If courts are in the business of promoting personal autonomy, shouldn’t marriage be open to every group that has a moral claim?

If the state is not allowed to distinguish among different types of personal relationships, then it has no ability to identify the social benefits of any type of private sociability. The district court noted this danger in the Abercrombie decision. “Once the link between marriage and procreation is taken away, and encouraging a socially desirable family structure is deemed irrational, there is no rational limiting principle for other types of relationships” to be excluded from marriage. This is not a slippery-slope argument. It is a matter of the limits of language.

This is a narrow ground for rulings in the marriage cases. It leaves unaddressed the important questions of what marriage is and of how it should be defined in law. That is precisely why the Court should find this approach attractive.