The influence of the categorization process is investigated in the context of conceptions of categories as fuzzy sets, represented by prototypes. The experiment specifically considers two social categories corresponding to Italian regional groups: for each of these, stimuli were constructed which represented the respective categories in different degrees. We hypothesized that the accentuation of the degree in which individual exemplars represent a category, obtained by the addition of an attribute of the category, produces the effect of accentuating inter-category differences and intra-category similarities. Our results confirm the hypothesis regarding differentiation, but not that regarding assimilation. The effect of differentiation is strong at the category borders, i.e. for the less representative exemplars.
A final issue considered is the theoretical problem of distinguishing, within the prototype of a category, between stereotypical attributes and those which permit categorial identification of individual cases.