Self‐categorization theory hypothesizes that group polarization arises from an intragroup process of conformity to apolarizedin‐group norm, i.e. a norm more extreme than the mean of members' initial opinions but in the same direction. It argues that the in‐group norm is embodied in the prototypical response of members, which is not necessarily their average position. Polarization occurs when the prototype is more extreme than the mean in the same direction and when mutual influence leads members to conform to the norm. Convergence on the mean occurs when the prototype coincides with the mean position. Whether the prototype and the mean coincide or differ depends on whether group responses are moderate or extreme in terms of the comparative social context. A computer simulation (Study 1,N= 1000 groups) and two semi‐naturalistic group discussion studies (Studies 2 and 3,Ns= 32 and 44 groups respectively over five items) were conducted to test this analysis. All three studies found significant correlational support for the hypothesis (H1) that in‐group prototypes become more polarized as the group becomes more extreme in the social context and Studies 2 and 3 also found a significant empirical correlation (H2) between polarization and the extent to which group members conformed to a polarized