Previous studies comparing ideological groups have been restricted to tests of between-group differences in the means of relevant political psychological variables, thereby neglecting group differences in the variances, meanings and nomological networks of the tested variables. A first exploratory study used data from the European Social Survey (N = 7,314) comparing groups of political party members on the basis of their scores on a self-placement leftright scale. The second study (N = 69) constituted an in-depth test for the presence of differences between samples of political activists of moderate parties, communists, anarchists and right-wing extremists. The results revealed that there is a fair amount of heterogeneity within left-wing and right-wing extremists, indicating a substantial amount of within-group variance of social attitudes, values and prejudice. Moreover, the extremist ideologies are best approached as distinct ideologies that cannot be reduced to extreme versions of moderate ideology, and differences in the meanings and nomological networks of the various extremist ideologies were also obtained. It is erroneous to consider members of extremist groups as being all alike. The findings obtained from samples of political moderates are not a particularly solid basis for theories about extremism.