Research has shown that undergraduate research experiences can have substantive effects on retaining students in science, technology, engineering and mathematics (STEM). However, it is impossible to provide individual research experiences for every undergraduate student, especially at large universities. Course-based undergraduate research experiences (CUREs) have become a common approach to introduce large numbers of students to research. We investigated whether a one-semester CURE that replaced a traditional introductory biology laboratory course could increase retention in STEM as well as intention to remain in STEM, if the results differed according to demography, and investigated the possible motivational factors that might mediate such an effect. Under the umbrella of the Authentic Research Connection (ARC) program, we used institutional and survey data from nine semesters and compared ARC participants to non-participants, who applied to ARC but either were not randomly selected or were selected but chose not to enroll in an ARC section. We found that ARC had significant effects on demographic groups historically less likely to be retained in STEM: ARC participation resulted in narrowing the gaps in graduation rates in STEM (first vs continuing-generation college students) and in intention to major in STEM [females vs males, Persons Excluded because of Ethnicity or Race (PEERs) vs non-PEERs]. These disproportionate boosts in intending STEM majors among ARC students coincide with their reporting a greater sense of student cohesiveness, retaining more interest in biology, and commenting more frequently that the course provided a useful/valuable learning experience. Our results indicate that CUREs can be a valuable tool for eliminating inequities in STEM participation, and we make several recommendations for further research.