High school students from lower-socioeconomic status (SES) backgrounds are less likely to enroll in advanced mathematics and science courses compared to students from higher-SES backgrounds. The current longitudinal study draws on identity-based and expectancy-value theories of motivation to explain the SES and mathematics and science course-taking relationship. This was done by gathering reports from students and their parents about their expectations, values, and future identities for science, technology, engineering, and mathematics (STEM) topics beginning in middle school through age 20. Results showed that parental education predicted mathematics and science course taking in high school and college, and this relationship was partially mediated by students' and parents' future identity and motivational beliefs concerning mathematics and science. These findings suggest that psychological interventions may be useful for reducing social class gaps in STEM course taking, which has critical implications for the types of opportunities and careers available to students. [For the corresponding grantee submission, see ED578803.]