We’re far enough past the days of sex-based benefits laws, alcohol laws, and public-university admissions requirements that the Supreme Court no longer hears a lot of cases about laws that expressly apply different rules to women as opposed to men. But on the day after our nation voted down the first female major-party presidential nominee in favor of a man who has made both a business and a pastime out of objectifying women, the Supreme Court heard a case that concerns a provision of federal immigration law that explicitly treats men and women differently. As the country prepares for the presidency of a man whose views on gender roles seem to have been fixed in the 1950s, Wednesday’s Supreme Court case, Lynch v. Morales-Santana, provides a small but meaningful snapshot of how sex-based laws written in that very era are viewed by the branch of government most likely to provide a check on the new president’s power.

What’s at issue is a provision of immigration law, written in 1940 and revised in 1952, that treats unmarried mothers differently from unmarried fathers in determining when they can pass their U.S. citizenship on to their kids: A mother passes her U.S. citizenship if she has lived in the U.S. for one year. By contrast, a father seeking to pass on citizenship must have lived in the U.S. for 10 years, including five after the age of 14.

Applying this law, the government claims in Wednesday’s case that Luis Morales-Santana, who was born in the Dominican Republic in 1962 to an unmarried Dominican mother and American father, did not receive his father’s U.S. citizenship because his father left the United States shortly before his 19th birthday and accordingly missed (by just three weeks) the requirement that he live in the U.S. for five years after the age of 14. (For children born after 1986, Congress changed the law to narrow—but not eliminate—the discrepancy in treatment between mothers and fathers.)

Morales-Santana, seeking to establish citizenship to avoid deportation, charges that the law unconstitutionally discriminates based on sex because if you switched the sexes of his parents, Morales-Santana would be a U.S. citizen.

If this distinction strikes you as arbitrary, you’re not alone. After a subdued start to the argument (justices apparently are no more immune to post-election hangovers than the rest of us), Justices Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor, and Elena Kagan take turns coming up with different ways to tell the government’s lawyer, Deputy Solicitor General Edwin S. Kneedler, that they think the law is silly.

The government claims that Congress wanted to ensure that the parent has sufficient connection to the United States to justify letting the child inherit citizenship. The four liberal Justices aren’t buying it. “Why aren’t men and women who are parents similarly situated with respect to their affiliation, their attachment to U.S. values?” Justice Ginsburg wants to know, opining further that “there’s no reason to think a man has less of a sense of U.S. belonging than a woman.” Kneedler pivots to a discussion of one way men and women are differently situated: the mother’s presence at birth. Justices Kagan and Sotomayor are unimpressed. Justice Breyer is visibly frustrated. Kneedler launches into a comparison of unmarried fathers to married fathers, but the justices keep returning to the basic distinction that the law makes between unmarried fathers and unmarried mothers.

What’s really behind the law, Morales-Santana’s attorney Stephen A. Broome tells the court, are stereotypes about gender roles in parenting. Morales-Santana’s brief pointed to statements of executive officials around the time of the law’s enactment opining that “almost invariably it is the mother who concerns herself” with the child and referring to the mother as the child’s “natural guardian.” Justice Ginsburg shares this concern, noting that at the time that this law’s relevant provisions were enacted, “laws just put mothers and children not born of the marriage together, and separated fathers from their children.” When Kneedler retakes the podium for rebuttal, Justice Breyer lectures him: “Today there are lots of fathers who do look after their children.”

The last time the court decided a case concerning differential treatment of men and women in federal immigration law, in the 2001 case of Nguyen v. INS, a sharply divided court ruled for the government—with both the women then on the court, Justices Sandra Day O’Connor and Ginsburg, in dissent. In Nguyen, the issue was whether Congress could require unmarried fathers to take extra steps (not required of mothers) to establish parentage in order to pass on their U.S. citizenship to their children.

Kneedler brings up Nguyen three times in the first five minutes, but Justice Anthony Kennedy waves it away as concerning a different question: The logic of Nguyen is that everyone knows the mother is the mother because she had to have been there at birth; not so for the father. By contrast, the question of how long it takes a person to established a connection to the United States is about values, not biology.

A lot has happened regarding women and the law since 2001. The court in 2007 read a federal employment discrimination statute as barring the claims of Goodyear employee Lilly Ledbetter, who suffered 19 years of pay discrimination based on sex, because Ledbetter filed suit too late; Congress rebuked the court by amending the law. There are three female justices now, and as Dahlia Lithwick has noted, their voices have become increasingly powerful, particularly when issues affecting women (such as abortion) are at stake. Indeed, earlier this year, the court struck down two Texas abortion-clinic regulations in the biggest win for abortion-rights supporters since 1992.

The court faced the exact issue in Wednesday’s case in 2011—and couldn’t resolve it, splitting 4–4 because Justice Kagan was recused. A reader of tea leaves might say that the prior deadlock bodes well for Morales-Santana: If there were four votes on his side without Justice Kagan, then her presence should get him to five, as she makes clear early in the argument that she doesn’t think Congress had a good reason for the sex-based distinction.

We can assume that the three other progressive justices voted against the law in 2011; who was the mystery fourth vote? Justice Kennedy turns in a fair audition for the role, first expressing skepticism about the relevance of the 2001 Nguyen case and then focusing the rest of his questions on the issue of what the court should do if it strikes down the classification—“level up” unmarried fathers to the same favorable treatment of unmarried mothers or “level down” unmarried mothers to the stricter requirements applicable to unmarried fathers?

Here the terrain gets rockier for Broome, who can’t seem to satisfy Chief Justice John Roberts’ questions about the proper focus of the remedy analysis. But as the justices bear down on the question of remedy, it seems increasingly clear that most of them have accepted that it’s unconstitutional to treat mothers and fathers differently as to the length of U.S. residency required to pass on citizenship. The only question appears to be how the court will implement that decision.

In light of the legal and cultural evolution on sex discrimination in the past 15 years, it’s not surprising that the court is taking a hard look at sexism and stereotyping in a mid-20th-century immigration law. What remains to be seen is whether, after this week’s election, that issue continues to be a priority for the other branches of government.

Disclosure: Scott Michelman is senior staff attorney at the American Civil Liberties Union of the Nation’s Capital. The national ACLU filed an amicus brief supporting Morales-Santana. The author was not involved in the writing of that brief. The views stated here are the author’s own.