Intergroup comparison may strengthen the beliefs of group members. The resultant belief polarization, according to self‐categorization theory (Turner, 1987), is mediated by the perceptual extremitization of the in‐group norm. To test the theory, 51 Christian believers and 52 atheists completed a belief questionnaire twice. Firstly, they estimated the typical beliefs of the in‐group (in‐group comparison condition) or out‐group (out‐group comparison condition), or indicated their own beliefs (self condition). The effects of comparison were tested by later asking all subjects to express their own beliefs on the same questionnaire. Only the in‐group condition produced polarization. As predicted, the polarization was mediated by the extremitization of the in‐group norm among atheists. Among believers, contrary to the prediction, the polarization was associated with the moderation of the in‐group norm, suggesting that the believers were responding as individuals concerned with their personal identities rather than as group members concerned with their