This study addresses closing the academic performance gaps between African American and Latino students and their White counterparts, and between girls and boys in mathematics. Of the various models and theories of these social inequalities that have been advanced in literature, one particularly compelling line of research concerns the idea of "stereotype threat"--the apprehension individuals experience when confronted with a personally relevant stereotype that threatens their social identity or self-esteem. A number of research programs suggest that interventions aimed at reducing stereotype threat can attenuate its effects in school-based contexts, yielding significant gains in test scores. Specifically, individuals can manage threatening situations by shoring up their self-concept--a strategy known as "compensatory self-inflation" or "self-affirmation." This study assessed the achievement impacts of this stereotype threat-reduction intervention relative to a control group condition on the test scores of nearly 1,000 middle-school students in the Madison (Wisconsin) Metropolitan School District. Findings show no evidence that the self-affirmations closed the Black-White achievement gap. However, gender and ethnic group moderated the impact of self-affirmation; the variation in achievement by gender and ethnicity appears to be slighter in the self-affirmation condition. The proposed work may have important implications for education policy and practice in the state. Moreover, the cost-effective intervention strategies described in this study--each involving simple 15-minute writing exercises--could easily be replicated and implemented across the United States, with the potential to help close the persistent achievement gaps within other school systems across the country. Tables and figures are appended.