THE GENERALIZATION OF EMERGENT RELATIONS IN EQUIVALENCE CLASSES - STIMULUS SUBSTITUTABILITY

After learning the conditional relations AB, BC, and CD, subjects passed the emergent relations tests BA, CB, DC, AC, BD, AD, CA, DB, and DA, showing the formation of two 4-member equivalence classes. The A, B, and C stimuli were nonsense syllables; the D stimuli were ''short'' or ''long'' lines. Generalization of reflexivity, symmetry, and equivalence was assessed by using intermediate line lengths for the training lines in the DD, DC, DB, and DA tests. Test lines closest to a trained line occasioned choice of the same comparisons as those occasioned by the trained line. These test lines, then, functioned as class members. As the disparity between the test and trained lines increased further, the choice of a given comparison decreased systematically. The overlap of the generalization gradients demonstrated the substitutability of stimuli in an equivalence class. The gradients narrowed when subjects could indicate that test lines did not belong to either class by the choice of a ''neither comparison.'' Gradient stability in the absence of reinforcement demonstrated the cohesiveness of the classes and their generalized extensions. Finally, these results suggest that the combined effects of primary generalization and equivalence class formation can account for the development of naturally occurring and natural kind categories.