This paper reports on "exclusion," a phenomenon demonstrated when a participant views an array of comparison stimuli, all but one of which has been defined in relation to a spoken sample stimulus (most often a dictated name). When a new undefined sample is spoken, participants immediately select the undefined comparison item without explicit training to do so. Exclusion has attracted interest from behavior analysts interested in analyzing emergent behavior and/or exploiting the potential of the procedure for teaching new behavior to persons with severe mental retardation. The importance of the research area has grown in recent years, as the phenomenon has been recognized also by researchers interested in the development of child language and comparative cognition. Since the first studies of exclusion, in the mid-1970s, we have learned much about the populations in which it appears, the types of stimuli that can enter into exclusion, and the contexts that affect performances. Yet a fundamental question remains unanswered: What is the origin of exclusion performance? Why do virtually all individuals demonstrate exclusion on the very first opportunity? A research group from the Shriver Center for Mental Retardation (USA) have recently developed a series of methodologically intricate, sometimes interconnected single-subject designs to answer these questions. This paper will describe work that has been completed thus far with individuals with severe mental retardation and a comparison group of typically developing children.

How do humans categorize items and events in their world, and what role does language play in this process? Such questions have been at the center of long-standing debate among philosophers and scientists from many eras, cultures, and disciplines. Fundamental to this debate are difficult questions of what is meant by the concept of a linguistic symbol. For instance, philosophers and scientists alike have articulated a conceptual distinction between the relations involved in rich linguistic symbols (true words) as compared to restricted paired associates (rote-learned associations). Yet it remains difficult to specify behaviorally what actually makes these two types of relations different. In this article, we describe methodology that may offer an operationalized model that allows empirical analysis of paired associate versus symbolic learning. Like any methodology, our approach alone does not pretend to fully resolve age-old questions. However, we intend to illustrate distinct advantages offered within this approach to scholars interested in symbolic functioning and its development.