In an article for Population and Development Review’s 50th anniversary issue, Aliya Saperstein challenges the long-standing assumption that demographic categories such as race, ethnicity, and gender are fixed throughout an individual’s life. She presents the growing body of evidence showing that identity fluidity—changes in how people identify over time—is both real and consequential.

Drawing on longitudinal surveys and administrative data, Saperstein reviews patterns of fluidity across gender, sexual orientation, and racial identity. While overall rates are modest, they are significantly higher among certain groups, such as multiracial individuals, sexual minorities, and adolescents navigating gender identity. Saperstein shows that these shifts cannot be dismissed as mere measurement error. Rather, they reveal how people adjust their identities in response to social context, cultural change, and stratification.

Saperstein argues that recognizing identity fluidity enriches demographic research by offering new insight into how inequality is reproduced. She calls for changes in data collection, survey design, and analytic strategies to better track identities across time.