Kelsey Chapple argues that in light of psychological research demonstrating that stereotypes flourish when groups are segregated on the basis of visible characteristics such as sex, all schools that are segregated on the basis of sex violate the Fourteenth Amendment’s guarantee against sex discrimination. Though the Supreme Court in United States v. Virginia left open the question of whether sex-segregated schools are constitutional, the Virginia court clearly articulated that the cornerstone of constitutional sex discrimination analysis is the inquiry of whether or not the state’s classification on the basis of sex serves to reinforce stereotypes. This Note offers an in-depth look at the leading psychological theories on stereotypes: examining what they are, how the operate, and in what conditions they thrive or are diminished. Situating this research in the history of sex-segregated schooling and the current proliferation of single-sex programs, this Note argues that single-sex schools, simply by virtue of segregating boys from girls, serve to perpetuate stereotypes about how males and females should behave, to what they should aspire, and what talents and capabilities they possess. Thus single-sex schools present a constitutional problem: by their very nature they violate the anti-stereotyping principle articulated by the Supreme Court’s sex discrimination jurisprudence and as such, violate the Fourteenth Amendment.