Concepts are central to human cognition, structuring our thoughts and allowing us to generalize our experiences, predict the future, and control our surroundings. The study of concepts in cognitive psychology has largely focused on categorization of individual objects based on descriptive features, e.g., a bird is an animal with wings and a beak (Rosch, 1973). In this respect it has diverged from other fields of cognitive science, such as linguistics, which focuses on argument structure and composition, Yet a significant new thread of research has emerged in the psychology of categorization, re-examining the role of relational structures in concept representation (Markman, & Stilwell, 2001; Gentner & Kurtz, 2005). This emerging field studies stable representation of relations (relational categories), e.g. X is visiting Y, and natural categories defined by the role objects play in events, e.g., a guest is a person who is visiting somebody else, or a catalyst is an entity that precipitates a change of state. This perspective unifies research on categorization with other areas in cognitive science in which relational representations have long been central: analogical and causal reasoning (Gentner, Holyoak & Kokinov, 2001), scripts and schemas (Shank, 1982), and lexical semantics (Dowty, 1979), amongst others.