A test of the discrimination account in equivalence class formation

An equivalence class is typically established when a subject is taught a set of interrelated conditional discriminations with physically unrelated stimuli and additional, untaught, conditional discriminations are then demonstrated. Interestingly, and perhaps counter-intuitively, the relations among the stimuli within such a class are not necessarily equal. Rather, some members of the class are differentially related to other class members. There are two opposing theories of why relations among stimuli are unequal: nodal and discrimination accounts. The former suggests that delayed emergence is due to nodal distance (i.e. the number of intervening stimuli) while the latter suggests that delayed emergence is an artifact of the training procedure itself. The current paper reports on the first empirical study that directly compares these theories. Forty participants were given training designed to establish two 5-member equivalence classes. Participants’ response speed supported the nodal account, thus suggesting that unequal relations among equivalence class members is not an artifact of the training procedure.