A central idea in social psychology is that people can construe other people in terms of two types of mental representations: social categories (e.g. male) and attributes (e.g. intelligent). It is assumed that assigning a person to a social category (i.e. social categorization) is one of the most important causes of stereotyping. However, no theory has yet successfully explicated the properties that distinguish social categories from attributes and how those distinct properties may cause stereotyping. We show that an interpretation of social categories as mental representations that strongly exclude other mental representations (i.e. 'excluders') can explain how social categories may cause stereotyping. In addition, we present computer simulations that implement the assumed principles in a connectionist model where social categories are interpreted as nodes with strong inhibitory links. We argue that our model solves fundamental ambiguities in social categorization theories and unifies these theories with connectionist models of person perception.