College students are often urged to "find their passion," but if students believe that passions or interests are fixed, they may not develop interest in fields beyond the academic identity with which they enter college. Can a brief intervention that portrays interests as developable, not fixed, boost interest, and even grades, in mandatory math and science coursework among students who do not identify as a "math or science person"? This would be especially significant because college provides the foundation for developing skills and interests that guide later professional paths. After a successful pilot study at a small liberal arts college (N = 175), we conducted a randomized, controlled field-experiment with matriculating first-year undergraduates (N = 580) in the school of arts and social sciences of a large university. Students completed a 30-min growth-theory-of-interest (vs. control) online module before starting school. At the end of their first and second semesters, they reported their interest in their two required first-year math/science courses. Official final grades were obtained at the end of the year. As predicted, among those who entered college less identified with math and science, the intervention (vs. control) increased interest and final grades in both first-year math/science courses (one conditional effect was marginal). The results suggest that by representing interests as not merely "found" but as having the potential to grow, colleges can encourage the development of skilled, interdisciplinary scholars.