Do contingency-conflicting elements drop out of equivalence classes? Retesting Sidman's (2000) theory

Sidman's (2000) theory of stimulus equivalence states that all positive elements in a reinforcement contingency enter an equivalence class. The theory also states that if an element from an equivalence class conflicts with a reinforcement contingency, the conflicting element will "drop out" of the class. Minster et al. (2006) found evidence that a conflicting reinforcer does not drop out of an equivalence class. To explain their results, they proposed that the reinforcer enters the class via pairing after conditional relations have been established, and when there is a conflict between the class and the contingencies, conditionally related elements that have a longer history in the class and that were brought in via reinforcement will exert stronger control. In the current study, stimulus-reinforcer relations were established before conditional relations to assess the role of developmental order of stimulus relations on conditional-discrimination performance. The results replicate the findings of Minster et al. (2006) but suggest that developmental order may not be an important factor contributing to maintained accuracy on baseline conditional relations. An interpretation of "dropping out" in terms of differentiated subclasses is discussed. The relevance of the results to the phenomenon known as the differential outcomes effect is also discussed.