This experiment investigated the differential effects of dispositional (gender) and situational (armband) group membership on in-group favoritism. Subjects met a confederate either similar or dissimilar in disposition and/or situation in an interpersonal setting. The proximity between the subject and the confederate, as well as the subject’s memory about the confederate, were the dependent measures. In addition, the ecological validity of studies that employ hypothetical situations in their methodology was examined. The results indicated that similarity of both disposition and situation resulted in the highest levels of favoritism. Memberships based on disposition, as compared with situation, resulted in greater favoritism. The results also demonstrated the need for ecological validity in research, as significant differences were found between actual and hypothetical settings.