It is too soon to predict how the U.S. Supreme Court will deal with the issue of same-sex marriage, but history has already been made.

The Department of Justice argued last week that in the context of marriage (and everywhere else), courts should treat discrimination on the basis of sexual orientation with essentially the same hostility they apply to discrimination on the basis of race and sex. To understand the importance of that argument, detailed in a brief opposing a 2008 California law banning same-sex marriage, we need to step back a bit.

Whenever the government treats some people differently from others, it may be acting unfairly, and its action can be challenged as discriminatory. The Supreme Court has responded to this problem by ruling that under the Equal Protection Clause of the 14th Amendment to the Constitution, the government can do whatever it likes so long as its action is “rational.”

This is not meant to be a demanding test. If the government says that people younger than 16 can’t drive, that the antitrust laws don’t apply to labor unions or that small companies are exempt from pollution-control requirements, there is no constitutional problem, because in each of these cases, the government can be said to be acting rationally.

In a few areas, however, the court requires the government to show far more than mere rationality. In these areas, the government is subject to “heightened scrutiny,” in the sense that the courts will require a convincing demonstration that the discrimination is genuinely necessary to promote an important social goal.

Heightened Scrutiny

Under heightened scrutiny, racial discrimination is usually struck down. If the government treats white people better, or worse, than African-Americans, it has to explain that the difference is required for some neutral reason, not involving prejudice, stereotyping or hostility.

It took many years, and a lot of vigorous advocacy (above all from then lawyer and now Justice Ruth Bader Ginsburg), but the court was eventually convinced that discrimination on the basis of sex should similarly be subject to heightened scrutiny.

The basic idea is that women have faced a long history of unfair stereotyping and have lacked the political power of men, so sex discrimination is likely to reflect prejudice. As Justice William Brennan wrote for the court 40 years ago, such discrimination tended to put women “not on a pedestal, but in a cage.” Whenever the government discriminates on the basis of sex, it will run into serious constitutional trouble.

Until very recently, it would have been adventurous, and possibly even absurd, for anyone to suggest that the court should apply heightened scrutiny to discrimination on the basis of sexual orientation. In these circumstances, it is historic that the Department of Justice has so argued, especially in the context of marriage.

In 2011, Attorney General Eric Holder and President Barack Obama initially signaled their acceptance of the argument in favor of heightened scrutiny, but it wasn’t until late last month that the department formally made the argument to the Supreme Court. It did so by asking the court to strike down the Defense of Marriage Act, which bans the federal government from recognizing same-sex marriages.

In an even more important and far-reaching step, the department suggested the court should invalidate the California law, enacted as a result of a ballot initiative, that denies same-sex couples entry into the formal institution of marriage while allowing them to become domestic partners.

The department’s brief says that heightened scrutiny is appropriate in light of the history of discrimination against gay men and lesbians, the absence of a relationship between sexual orientation and a person’s ability to contribute to society, the fact that the group is unified by a distinguishing characteristic, and the limited ability of gay men and lesbians to protect themselves in the political process.

California’s Law

The department insists that California’s law cannot survive heightened scrutiny because the purported justifications -- that it promotes responsible child-rearing, for example -- aren’t adequately supported by either logic or evidence.

By emphasizing heightened scrutiny for discrimination against gay men and lesbians, the department deftly avoids the objection, raised by some critics, that if states are required to recognize same-sex marriages, they will have to recognize polygamous ones as well. Indeed, the department doesn’t even contend that all states must recognize same-sex marriages. If the court accepted the department’s argument, it would be committed only to the narrow proposition that states can’t forbid same-sex marriages if they authorize same-sex domestic partnerships.

Nonetheless, if the department is right, it would not be so easy to avoid accepting a much broader proposition, which is that states engage in impermissible discrimination if they forbid gay men and lesbians from entering into marital relationships. That proposition doesn’t yet command a social consensus, but the nation, no less than the executive branch, is unquestionably moving in its direction.

Whatever the Supreme Court ultimately rules, the arc of American history is bending, and it is bending toward justice.

(Cass R. Sunstein, the Robert Walmsley University Professor at Harvard Law School, is a Bloomberg View columnist. He is the former administrator of the White House Office of Information and Regulatory Affairs, the co-author of “Nudge” and author of “Simpler: The Future of Government,” to be published in April. The opinions expressed are his own.)

To contact the writer of this article: Cass R. Sunstein at csunstei@law.harvard.edu.

To contact the editor responsible for this article: Katy Roberts at kroberts29@bloomberg.net.