Professionals in science, technology, engineering, and math (STEM) are often stereotyped as geniuses and nerds (e.g., socially awkward). These stereotypes may demotivate some individuals from pursuing or remaining in STEM. However, these beliefs may enhance motivation among individuals who feel that they fit in with the stereotype. Guided by balanced identity theory and expectancy-value theory, our study investigated the effect of trait-based stereotypes about people in STEM among a sample of 256 U.S. high school students (M[subscript age] = 16, 59% girls, 65% Asian, 15% Latinx, 10% White). We assessed students' trait-based nerd and genius stereotypes about STEM and related self-concepts as well as their STEM motivation (competence and value beliefs). Consistent with balanced identity theory, the effect and direction of endorsing nerd-genius stereotypes was moderated by a student's own self-concepts. Endorsing stereotypes was negatively related to motivation--but only among those low in the related self-concept. Among those high in related self-concepts (e.g., high in nerd-genius self-concept), endorsing STEM stereotypes (e.g., STEM is for geniuses) was unrelated to STEM motivation. Girls, underrepresented students of color, and potential first-generation college students may especially be negatively affected by the stereotypes due to a greater likelihood that these stereotypes will be incongruent with their self-concepts. Thus, trait-based stereotypes about people in STEM may perpetuate current gaps in STEM.