This study examines how young adults evaluate gender-based social inclusion and exclusion from academic peer groups. Participants included 199 college students (M[subscript age] = 19.18; SD = 1.37, Range = 18-25), who made judgments about the acceptability of gender-based social exclusion of female and male peers from a Physics group (a stereotypically masculine field) and bystander responses to gender-based social exclusion. Equitable attitudes and acceptability of gender-based social exclusion were examined as predictors of bystander responses to social exclusion. Findings showed that participants were less likely to see the exclusion of a female as acceptable compared to the exclusion of a male. However, regarding expected bystander intervention, models differed based on condition (exclusion of a male compared to exclusion of a female peer). Specifically, in the female exclusion condition, participants' gender predicted equitable attitudes, which then predicted expected bystander intervention and ratings of acceptability. In the male exclusion condition, equitable attitudes predicted acceptability, which then predicted expected bystander intervention. These findings have important implications for understanding how to shape inclusive campus climates.