A large body of research suggests that extremists make more accentuated (‘black and white’) judgements than moderates. This phenomenon has been explained in terms of individual differences associated with extremism or as the product of general processes of social judgement. Self-categorization theory suggests that extremists represent the world in relatively polarized terms only because, and to the extent that, similar others are more similar to them and different others are more different from them than is the case for moderates. This analysis is tested and supported in three experiments (Ns = 61, 101, 69) which (a) manipulate subjects' extremity and reproduce standard accentuation effects, and (b) change the configuration of the comparative features comprising the judgmental context and reverse these effects. These findings are inconsistent with individual difference explanations of accentuation and extend previous social judgemental theorizing. Implications for the conceptualization of extremism are also discussed.