With small liberal arts colleges (SLACs) producing an increased percentage of STEM graduates, whose voice resonates most regarding science, technology, engineering, and mathematics (STEM) degree persistence? Chemistry professor? Dad? Best friend? This study investigated if undergraduate student perception of close social subgroup influences affected motivation characteristics (i.e., self-efficacy, STEM interest, and outcome expectations) and STEM major persistence. STEM majors (N = 295) across each college year at a Midwestern, 4-year liberal arts institution were surveyed regarding subgroup influence on motivation toward STEM degree attainment. A hypothesized structural equation model was tested to determine the subgroups that STEM majors perceived affected STEM motivational factors and degree persistence. Results indicate that STEM professors and friends have significant explicit and implicit effects on STEM degree persistence. The model accounted for a significant amount of variance in STEM interest, self-efficacy, and persistence for the overall sample and for underrepresented group members. Educational implications for college administrators and professors, and future research directions are discussed.