Stimulus class formation and concept learning: establishment of within- and between-set generalization and transitive relationships via conditional discrimination procedures.

Three students with moderate mental retardation were taught a complex stimulus class with a two-choice conditional discrimination procedure applied across eight 10-member stimulus sets. Each set was composed of five age-appropriate and five age-inappropriate examples of clothing, accessories, and leisure items (e.g., a Walkman radio). Discrimination training was programmed serially across each set, and generalization probes were conducted concurrently among all sets. Generalization probes consisted of unreinforced conditional matching trials with comparison items being drawn from (a) the set undergoing training (within-set probes), (b) sets not undergoing training (between-set probes), and (c) both sample and comparison items from different sets (transitive stimulus control probes). Results indicate that within-set generalization, between-set generalization, and transitive stimulus relations controlled responding by all 3 students for items that had been contingently associated with reinforcement. However, items that gained control of responding through within-set and between-set generalization alone (i.e., not acquired through contingent reinforcement) remained at baseline levels during transitive stimulus control probes. Results are discussed in terms of a taxonomy of multiple sources of stimulus control that underlie socially defined and maintained stimulus classes.