According to Optimal Distinctiveness Theory and Self-Categorization Theory possible self-definition as a member of contextually moderately distinctive social categories should be more central to identity than association with very general or with highly unique social categories. Data from a large scale cohort survey of 4156 18–21-year-olds were analysed to test this hypothesis. Respondents self-reported political affiliations were classified according to the social popularity (size and support) accorded each party The data are consistent with the hypothesis that minority political parties should provide more central and important bases of social identity Supporters of minority parties showed greatest commitment, perceived their parties to be more representative of themselves, and were less likely to simply conform to parental political views. Minority supporters were also significantly more likely to discuss politics with their friends. Additional data from Scotland confirmed that identification with Scotland was more strongly associated with support for the Scottish Nationalist Party than with support for other minority or majority parties. These data provide convergent evidence that minority parties may attract members through their capacity to provide a meaningful social identity.