This article argues that in-group favoritism occurs on positive and negative dimensions only when the dimensions of comparison provide an appropriate and meaningful basis for self-other definition, that is, when traits comparatively and normatively fit in-group-out-group categorizations. Three studies are reported in which groups were evaluated on positive or negative traits that varied in their degree of normative fit to in-group and out-group identity. In line with predictions, fit rather than stimulus valence was the crucial determinant of (a) in-group favoritism and (b) absolute level of differentiation between groups. Implications of the findings for explanations of positive-negative asymmetry and broader understandings of intergroup discrimination are discussed.