This study utilized John Holland's personality typology and the Social Cognitive Career Theory (SCCT) to examine the factors that may affect students' self-selection into science, technology, engineering, and mathematics (STEM) majors. Results indicated that gender, race/ethnicity, high school achievement, and personality type were statistically significant factors in increasing or decreasing a student's odds of enrolling in a STEM major. Specifically, students with a strong investigative personality were more likely to enroll in STEM majors, while those with a strong artistic personality or enterprising personality were less likely to do so. Males with a strong social personality also tended not to choose STEM majors, though social personality had a positive effect on whether females chose STEM majors. Implications of the findings for policymakers, educators, and administrators were explored.