Why are some fields and occupations more diverse than others? In this review, we describe a conceptual framework--the field-specific ability beliefs (FAB) model--that provides a promising answer to this question. This model proposes that gender and racial/ethnic imbalances in a field or occupation result in part from the confluence of two beliefs: (a) the belief that success in that context requires high levels of intellectual ability ("brilliance") and (b) the cultural stereotype that associates intellectual ability with (White) men more than other groups. We describe the FAB model and detail evidence for it, including evidence that the beliefs at its core are present even among children. We conclude by highlighting open questions.