A foundation in science, technology, engineering, and mathematics (STEM) education is critical for students' college and career advancement, but many U.S. students fail to take advanced mathematics and science classes in high school. Research has neglected the potential role of parents in enhancing students' motivation for pursuing STEM courses. Previous research has shown that parents' values and expectancies may be associated with student motivation, but little research has assessed the influence of parents on adolescents through randomized experiments. Harackiewicz, Rozek, Hulleman, and Hyde (2012) documented an increase in adolescents' STEM course-taking for students whose parents were assigned to a utility-value intervention in comparison to a control group. In this study, we examined whether that intervention was equally effective for boys and girls and examined factors that moderate and mediate the effect of the intervention on adolescent outcomes. The intervention was most effective in increasing STEM course-taking for high-achieving daughters and low-achieving sons, whereas the intervention did not help low-achieving daughters (prior achievement measured in terms of grade point average in 9th-grade STEM courses). Mediation analyses showed that changes in STEM utility value for mothers and adolescents mediated the effect of the intervention on 12th-grade STEM course-taking. These results are consistent with a model in which parents' utility value plays a causal role in affecting adolescents' achievement behavior in the STEM domain. The findings also indicate that utility-value interventions with parents can be effective for low-achieving boys and for high-achieving girls but suggest modifications in their use with low-achieving girls.