The Equal Protection Clause requires that heightened scrutiny apply to sex classifications. The standard account of the Supreme Court’s sex equality jurisprudence assumes that sex is a matter of biology. Sex as biology has long served as a roadblock to sex equality, with reasoning about the body shielding sex discrimination from scrutiny by covering for stereotypes and masking the social sources of inequality. More recently, the biological understanding of sex has featured prominently in decisions rejecting claims brought by transgender plaintiffs excluded from sex-segregated spaces like athletics and bathrooms. With a circuit split in these cases, it is only a matter of time before the meaning of sex under equal protection reaches the Court. Transgender rights — and sex equality itself — hang in the balance. 

This Article shows that the standard account of sex as biology is wrong. It reveals how the role of biology in the Supreme Court’s sex equality jurisprudence has been far more contested, partial, and, ultimately, fleeting than the standard account acknowledges. The key decision at the doctrine’s inception explained why heightened scrutiny should apply to sex classifications without referencing biology and by relying on factors — immutability, visibility, history of discrimination, and political power — that do not rest on any biological aspect of sex. Subsequent cases that have been viewed as the height of biological reasoning were never controlling on this question. Biology has only served as the basis for the Court’s holding one lone time, with the support of a bare majority, and in a decision that is now seriously in doubt. 

With sex no longer bound to the body, this Article recovers an alternative conception of sex running through the jurisprudence: sex as a social class. This idea recognizes that sex, like race, is a product of social forces. On this view, sex discrimination can no longer seek refuge in the body, and would instead face exacting scrutiny for a legitimate justification related to the social function that sex serves. The social conception of sex not only avoids the harms of sex as biology, but can unite transgender and cisgender women as a class for equal protection purposes (so too transgender and cisgender men), showing a way forward in highly contested transgender rights cases. This new account of sex equality law provides the tools to achieve the Constitution’s promise for a more equal world for transgender persons — and for us all.