California has followed Massachusetts into uncharted territory by requiring California schools to make sex-segregated facilities and activities available to members of both sexes. Those who advocated this move might not like all of the implications of what they have accomplished. Among the many likely casualties of these laws will be the logic of the Supreme Court’s equal protection jurisprudence, which protects females from suspect classifications in law.

Earlier this year, the Massachusetts Department of Education (MDOE) issued regulatory guidance for Massachusetts schools concerning a recent state statute that prohibits discrimination on the basis of gender identity. MDOE interprets the statute to give students a right to use the bathroom, locker room, and changing facilities that correspond to the gender with which those students identify, regardless of their biological sex.

The MDOE directive notes that some students might be uncomfortable disrobing with a member of the opposite sex, but insists, “This discomfort is not a reason to deny access to the transgender student.” The directive also encourages school administrators to discipline students who object to sharing a bathroom with a member of the opposite sex.

At the time, Andrew Beckwith and I argued that this development is part of a trend: judges and lawmakers are eliminating sexual distinctions from Massachusetts law, and driving out of public life anyone who perceives inherent differences between male and female. But the MDOE directive presupposes that many sex-segregated activities will persist; indeed, they will persist because at least some transgendered students want them to persist. A biological boy who identifies as a girl does so in part by using the girls’ bathroom.

Start your day with Public Discourse

Sign up and get our daily essays sent straight to your inbox.

MDOE did what the state legislature declined to do. The legislature had earlier enacted a statute prohibiting discrimination on the basis of “gender identity,” but pro-family forces in the Commonwealth defeated a provision in the original bill that would have given boys a right to use girls’ bathrooms and vice versa. Yet MDOE proceeded as if the provision had been enacted.

In this respect, and in this respect alone, the law recently enacted in California represents an improvement over the legal change in Massachusetts—at least it was promulgated democratically. The legislature passed, and the California governor signed, a statute giving students the right to choose which gender they will be in sex-segregated activities.

It should go without saying that to give boys a legal right to disregard their Y chromosomes when selecting a place to undress is to invite abuse. It should go without saying that this law is a menace to school-age girls and their families. It should go without saying that a child who feels alienated from his or her own sex ought to be encouraged and affirmed for who he or she is, rather than given a legal right to pretend to be something he or she is not. But alas, such evident truths no longer go without saying.

However, advocates for these laws may have attained more than they intended. These laws undermine a foundational premise of the Supreme Court’s equal protection jurisprudence, namely that sex is an immutable characteristic. If the bathroom laws in Massachusetts and California are grounded in truths about human nature and sexuality, then that premise is either false or irrelevant, and it is not clear on what basis the Supreme Court could continue to employ heightened scrutiny when reviewing laws that discriminate between men and women.

For four decades, the Supreme Court has reviewed laws that discriminate between men and women with extra attention. In Frontiero v. Richardson (1973) it struck down a statute that treated husbands of female military personnel less favorably than wives of male personnel for the purpose of distributing allowances and benefits. The Court declared sex to be a suspect classification. It reasoned that

since sex, like race and national origin, is an immutable characteristic determined solely by the accident of birth, the imposition of special disabilities upon the members of a particular sex because of their sex would seem to violate the basic concept of our system that legal burdens should bear some relationship to individual responsibility.

Therefore, the Court found classifications based on sex to be “inherently suspect,” and resolved to review them with heightened judicial scrutiny.

When someone is discriminated against because of immutable traits that do not affect that person’s suitability for the opportunity denied to him or her, the discrimination appears more likely than not to be unjust. The classic example of an immutable characteristic remains one’s race, and the best example of unjust discrimination remains a law requiring or allowing racial segregation.

Of course, one’s sex does make one more or less suitable for certain opportunities—e.g., being a mother—but not for others—e.g., practicing law. Where to draw the line between those sets of opportunities is sometimes a controversial question. But the immutability of sex has long been a given. It explains why the law reasonably discriminates in favor of women when it identifies the mothers of children. It also explains why a law prohibiting women from practicing law would be unjust. Under such a law, a woman would be disqualified for a reason that she cannot control and that does not render her unsuitable for the opportunity.

The immutability of sex has long been a foundation stone of the Court’s equal protection jurisprudence. In the landmark case United States v. Virginia, the Court affirmed that physical differences between men and women are “enduring”; that “a community made up exclusively of one sex is different from a community composed of both”; and that inherent differences between men and women are “cause for celebration.” But, the Court stated, those differences are not reasonable grounds for segregating the sexes arbitrarily.

Now education regulators in Massachusetts and lawmakers in California have declared sex or gender to be mutable. It is a characteristic to be denied, or even escaped from, if authentic self-definition requires it.

If gender is what counts, and if gender is mutable, then on what ground should references to sex and genetics not be eradicated from the law? And if that is the lesson to be learned from Massachusetts and California then why should the Equal Protection Clause be exempted from the leveling to which all other laws must yield? The Court treats sex-based classifications with higher scrutiny than classifications based on age, for example, in part because it views women as beings who are born with particular traits and genetic compositions that distinguish them from men and do not change with time.

Moreover, the remedial requirements of the Massachusetts and California laws might come into conflict with the requirements of equal protection. Consider again United States v. Virginia. In that case, the Court struck down on equal protection grounds the male-only admission policy of the Virginia Military Institute. The Court required VMI to demonstrate an “exceedingly persuasive justification” for the policy. VMI explained that its rigorous, “adversative method” of educating cadets would need to be made less rigorous to accommodate women. The Court conceded that VMI’s program of education would need to be altered, but insisted that because some women would be qualified for the program, this justification was not “exceedingly persuasive.”

On this logic, the remedy for unlawful sex discrimination is integration of the sexes for those activities to which men and women are equally suited and segregation of the sexes for those activities in which their differences are important. Women are born as women, and remain women throughout their lives. To treat them justly is to integrate them into the activities of men for which at least some of them can be well suited, while establishing or preserving accommodations for their natural differences.

By contrast, the gender-identity self-authorship that Massachusetts and California now must promote requires at least some segregation of the sexes for those activities to which some men and some women might be equally suited, such as athletic competition. If a member of the male sex identifies as a member of the female gender, and if the community owes to that person opportunities to express himself as a female, then he must have female-specific activities in which to participate. The mutability of gender, on this line of thinking, is a reason to keep men and women separate so that those who would change their gender identity can have opportunities to do so.

Meanwhile, the mutability of gender is also a reason to allow at least some biological boys the right to disrobe with biological girls. If a female is one who identifies as a member of the female gender, regardless of sex, then the biological boy who identifies as female really is a female, and belongs in the girls’ room. Whatever burdens this arrangement imposes upon the biological girls in the girls’ room are not sufficient reasons to shield them from this newly-recognized reality. And so, on this line of thinking, girls should bear the burdens of sharing bathrooms and locker rooms with some biological boys.

What became of the “the basic concept of our system that legal burdens should bear some relationship to individual responsibility,” which the Frontiero Court vindicated? What special responsibilities do school-age girls have to share bathrooms and locker rooms with boys who identify themselves as girls, or of competing with boys for positions on girls’ sports teams? This development seems to flip the Supreme Court’s equal protection jurisprudence on its head.