Massachusetts gets it right.

In its extraordinary decision in Goodridge v. Department of Public Health last month, the Massachusetts Supreme Judicial Court ruled that a prohibition on same-sex marriage violates the Massachusetts constitution. While the court drew some support from federal precedents, its decision was plainly grounded in previous interpretations of the state constitution and its distinctive guarantees of equality and liberty. The U.S. Constitution was not involved; it was state--not federal--law that formed the basis for this ruling. We should therefore celebrate Goodridge, not only because it ends a form of second-class citizenship for gays and lesbians but also because it exemplifies the federal system at its best.

The Massachusetts court's most important conclusion in Goodridge was that the state had failed to produce a "rational basis" for its refusal to allow same-sex couples to marry. The state had defended its prohibition of gay marriage principally by arguing that it sought to maintain a "favorable setting for procreation." But, given Massachusetts law, that is a ludicrous explanation. For well over a century, Massachusetts has held that a marriage need not be consummated to be valid. It even allows same-sex couples to have insurance coverage for assisted reproductive technology. In the court's words, the "Commonwealth affirmatively facilitates bringing children into a family regardless of whether the intended parent is married or unmarried ... and whether the parent or her parent is heterosexual, homosexual, or bisexual."

The state also defended its ban by claiming that it sought an "optimal setting for child rearing," which it defined as "a two-parent family with one parent of each sex." But this rationale is equally difficult to square with Massachusetts's laws and precedents, which recognize and attempt to protect many families that do not fit the traditional mold. For example, in 1983 the court ruled that homosexual orientation is not sufficient grounds for denying child custody in a divorce case; a 1999 decision, also involving child custody, emphasized that the child's best interests should take full account of the child's relationship with "de facto same-sex parents." Furthermore, Massachusetts ensures that adoption is available to married couples and same-sex couples alike. As the court concluded in Goodridge, the state's lawyers "offered purported justifications for the civil marriage restriction that are starkly at odds with" Massachusetts's distinctive effort to provide a "comprehensive network of vigorous, gender-neutral laws promoting stable families and the best interests of children." And, in any case, "the task of child rearing for same-sex couples is made infinitely harder by their status as outliers to the marriage laws," not least because of the significant economic and social benefits of marriage under state law.

Despite the decision's logic, it is tempting to criticize Goodridge as an illegitimate judicial intervention into a controversy that should be settled politically. And, to be sure, it would be preferable if gay marriage had been ratified by the Massachusetts legislature rather than the state supreme court. Judges, after all, should not lightly take sides in any "culture war," nor should they try to engineer social change by exploiting ambiguous constitutional provisions. (This was a serious problem in Roe v. Wade, where the U.S. Supreme Court used the due process clause as the basis for a broad ruling on behalf of the right to choose.) But these strictures against judicial activism lose much of their force if state judges are simply interpreting state law--as they were in this case--and if the ruling does not foreclose continuing debate within the state. And, in Goodridge, the Massachusetts court went out of its way not to foreclose further debate, taking the extraordinary step of delaying its decision for 180 days "to permit the Legislature to take such action as it may deem appropriate in light of this opinion." What does this mean? Well, the ruling says that the state constitution would not permit the state to bar "an individual from the protections, benefits, and obligations of civil marriage solely because that person would marry a person of the same sex." While this is open to interpretation, the court appears to be saying that the Massachusetts legislature could satisfy its constitutional duty by recognizing civil unions--so long as they provided the "protections, benefits, and obligations of civil marriage"--without necessarily calling them "marriage."

Moreover, should the state wish to reject Goodridge in its entirety, it could amend the Massachusetts constitution fairly easily. The process is time-consuming but less arduous than changing many state constitutions. To get on the ballot, it is enough for an amendment to be approved by merely one-quarter of state legislators in two successive legislative sessions and then to be ratified by a bare majority of Massachusetts voters in a statewide election. Such amendments are not unusual in Massachusetts, where the constitution has been changed 55 times since 1919--sometimes in response to actual or anticipated decisions of the state supreme court. Most prominently, the state constitution was amended in 1982 to authorize capital punishment when the state supreme court was expected to invalidate it. Massachusetts Governor Mitt Romney, along with many others, has already indicated his support for an amendment that would outlaw gay marriage. If the court's decision stands, it will be because these efforts to pass an amendment have failed, demonstrating that Massachusetts voters do not fundamentally object to gay marriage.

The reasonableness of the Massachusetts court's decision does not, however, mean the U.S. Supreme Court should follow suit now or in the near future. Quite the contrary. At the national level, judges ought to show caution in ruling on gay rights. To date, that is exactly what they have done--in marked contrast, it should be noted, to Roe v. Wade, in which the Court attempted to settle public debate over abortion with one bold stroke. Instead, during the last 15 years, the U.S. Supreme Court has entered the fray over gay rights exactly twice. In its 1996 decision in Romer v. Evans, the Court struck down a bizarre amendment to the Colorado constitution that banned the state and its cities and localities from outlawing discrimination against homosexuals. The Court's narrow ruling emphasized that this provision had the "peculiar property of imposing a broad and undifferentiated disability on a single named group." In Lawrence v. Texas, decided last summer, the Court took another small step, concluding that states could not punish people engaging in consensual sex. Demonstrating its sensitivity to the importance of procedural legitimacy, the Court stressed that anti-sodomy laws utterly lack popular support, as reflected by the fact that they have fallen into a "pattern of nonenforcement" in the few states where they remained on the books.

Meanwhile, the Court has declined to intervene in the gay rights debates over which the nation is most sharply divided. It has not said a word about same-sex marriage, child custody, discrimination by public employers, and "don't ask, don't tell." In fact, the U.S. Supreme Court has been at most a bit player in what appears to be a genuine revolution in the legal status of American gays and lesbians; it is the states that have taken the major strides. (For example, California, Connecticut, Hawaii, Massachusetts, Minnesota, Nevada, New Hampshire, New Jersey, Rhode Island, and Vermont, among others, now forbid employers to discriminate on the basis of sexual orientation.) And this is how it should be. An attempt by the U.S. Supreme Court to settle the same-sex marriage debate at this time would be disastrous, undoubtedly causing a heated public backlash and endangering the cause of gay rights itself. The genius of the federal system lies in the fact that, while requiring nationwide respect for certain rights, it allows the law to adapt to the states' diverse cultures, providing extensive room for experimentation and learning. This process should be allowed to run its course before the U.S. Supreme Court weighs in.

Of course, such state-level experimentation will lead to different laws, raising one obvious question: Must other states recognize same-sex marriages conducted in Massachusetts? If so, Massachusetts would be effectively setting marriage policy for the nation. And, indeed, the U.S. Constitution's Full Faith and Credit Clause does require each state to respect the "public Acts, Records, and judicial Proceedings of every other state." But, well before Goodridge was decided, Massachusetts law provided that out-of-state visitors could not marry in Massachusetts when the union would be invalid in their state of residence. In any case, Congress foresaw the ramifications of a Goodridge-like ruling in 1997 when it passed the federal Defense of Marriage Act, which expressly authorizes states to refuse to recognize same-sex marriages even if they are valid in the state where they were performed. And, even if the Defense of Marriage Act were at some point overturned, for over a century states have been permitted to refuse to recognize certain marriages--those between first cousins, say--that are valid in the state of the ceremony but inconsistent with their own public policy.

The Goodridge ruling makes sense in large part because it stems directly from Massachusetts law. But it also makes sense because of the moral issue it addresses: the right of gays and lesbians to participate in one of society's most important and cherished institutions. Civil marriage is the principal means by which people publicly affirm their commitment to one another, but its legal and social effects are much broader than that. Let there be no confusion on this point: Marriage is an institution created by governments, not by nature. (Some opponents of gay marriage emphasize that marriage is created and sanctified by God; but we are speaking here of civil marriages, not religious ones.) Its benefits are intensely material as well as symbolic, including rights to insurance benefits, health coverage in the event of death of a spouse, generally lower tax rates, hospital visitation in the event of serious illness, and many more.

The Massachusetts court was correct to say that the "marriage ban works a deep and scarring hardship on a very real segment of the community." In an important and even daily sense, the prohibition on same-sex marriage turns gays and lesbians into second-class citizens. There is thus a moral as well as a legal foundation for the Goodridge decision. Under the Massachusetts constitution, as under the U.S. Constitution, the core goal of the equality principle is to prevent the government from creating a caste system. As Justice John Harlan wrote in his lonely dissent to Plessy v. Ferguson, which found that providing separate facilities to segregate blacks was constitutional, "there is no caste" in America. The ban on same-sex marriage seems to violate that principle. Massachusetts Justice John M. Greaney, concurring separately in Goodridge, made this point explicitly, complaining of the "continued maintenance of this caste-like system."

Plausible as I find these arguments, however, they of course command nothing like universal agreement. But therein lies the beauty of Goodridge: To approve of it, it isn't necessary to be convinced that same-sex unions are legitimate and should be legalized across the country. It is necessary only to acknowledge that ours is a federal system and that reasonable people have made reasonable claims on behalf of those unions. In the 2000 campaign, Dick Cheney wisely emphasized that "different states are likely to come to different conclusions, and that's appropriate. I don't think there should necessarily be a federal policy in this area." Other states need not follow the route of Goodridge, but, regardless of how they resolve the issue, their choices will now be more informed, having had the opportunity to observe how well same-sex unions work in Massachusetts. Many Americans are seeing the Goodridge decision as a stirring tribute to individual rights. But, equally important, it is a remarkable tribute to U.S. federalism. We should celebrate it as such.

Cass R. Sunstein is a contributing editor at The New Republic.