This dissertation investigates how the numerical representation of women in small collaborative groups influences their learning in a college biology class for non-majors focused on sex, gender, and society. Prior to the start of the semester, the researchers divided students into all-male, all-female, or mixed-gender groups of varying sex ratios. The investigators then addressed how the identity-driven local ecology of these groups shaped student engagement and the emergence of scientific reasoning. Results from two separate but related studies demonstrated that all-female groups outperformed mixed gender and all-male groups across a range of behavioral, affective, and performance metrics. Yet, a narrower focus on the emergence of scientific reasoning roles and behaviors in all-female groups revealed striking variation among these groups. One all female group routinely used higher-order science reasoning moves, while another failed to perform academically, because students in this group struggled interpersonally. Data from semi-structured interviews suggest that all-female grouping does increase a sense of belonging in female-identified students--especially if they previously experienced invidious discrimination in science spaces and are thus vulnerable to social identity threat. However, the results also suggest that this type of grouping does not function as a panacea, and that additional social identities and lived experiences modulate whether single-sex grouping results in learning gains or deficits. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]