The present study proposes an extension to the phenomenon of ingroup favouritism, based on the hypothesis that judgments about ingroup members may be more positive or more negative than judgments about similar outgroup members. It contrasts predictions issued from the complexity-extremity hypothesis (Linville, 1982; Linville and Jones, 1980), from the ingroup favouritism hypothesis (Tajfel, 1982) and from Tesser's (1978; Millar and Tesser, 1986) attitude polarization model. Our main prediction, based on Social Identity Theory, is that judgments about both likeable and unlikeable ingroup members are more extreme than judgments about outgroup members. This phenomenon, coined the Black Sheep Effect, is viewed as due to the relevance that ingroup members'behaviour, as compared to that of outgroup members, has for the subjects' social identity. Three experiments supported our predictions. Experiment I additionally showed that inter-trait correlations were stronger for the ingroup than for the outgroup. Experiment 2 showed that the black sheep effect occurs only when the judgmental cues are relevant for the subjects' social identity, and Experiment 3 showed that levels of information about the target of the judgment were ineffective in generating judgmental extremity. Results are discussed in light of a cognitive-motivational alternative explanation to a purely cognitive interpretation of outgroup homogeneity.