EXPERIMENTAL ANALYSIS OF HUMAN BEHAVIOR BULLETIN 2007, 25, 1-5 BRIEF REPORT REARRANGEMENT OF EQUIVALENCE CLASSES AFTER REVERSAL OF A SINGLE BASELINE RELATION: INFLUENCE OF CLASS SIZE. Aline G. Folsta and Julio C. de Rose UNIVERSIDADE FEDERAL DE SÃO CARLOS Experimental studies of stimulus equivalence typically have two phases: the first teaches a set of baseline conditional discriminations, and the second tests for emergent relations. For instance, after a participant learned baseline conditional discriminations AB and BC, emergence of conditional relations BA and CB documents symmetry, emergence of relation AC documents transitivity, and emergence of relation CA documents combined symmetry and transitivity. Positive results on tests for these properties document the formation of equivalence classes (e.g., Fields, Verhave, & Fath, 1984; Sidman & Tailby, 1982). Studies often omit tests for reflexivity and, less frequently, take combined symmetry and transitivity as the sole evidence of equivalence. An important question about equivalence classes is their flexibility, i.e., to what extent classes can be altered after they have formed. Studies investigating this question typically have two additional experimental phases after the demonstration of equivalence. The first is a modification in the baseline, most often a partial reversal of the baseline conditional discriminations. For instance, after training of AB, Based on a paper presented by the first author in partial fulfillment of the requirements for an undergraduate degree in Psychology at Universidade Federal de São Carlos. Research supported by FAPESP (State of São Paulo Foundation for Research Support) Grant # 03/09928-4. The second author also benefited from a Research Productivity Grant from CNPq (National Council for Scientific and Technological Development). Send correspondence about this article to Julio C. de Rose, Departamento de Psicologia, Universidade Federal de São Carlos, Caixa Postal 676; 13565-905, São Carlos, SP; Brazil. E-mail: djcc@power.ufscar.br. and BC, with two samples and two comparison stimuli in each relation, and demonstration of two classes of equivalent stimuli (A1/B1/C1, and A2/B2/C2), a reversal of the original AB relation is trained, so that selections of B1 in the presence of A2 and of B2 in the presence of A1 now produce reinforcers. This reversal training is followed by another phase of equivalence tests, to verify whether classes modify according to the reversed baseline. From a purely logical perspective, in the test after reversal training the classes should rearrange to A1/B2/C2, and A2/B1/C1, according to the most recent training. Earlier studies, however, did not find the predicted rearrangement (Pilgrim & Galizio, 1995; Saunders, Saunders, Kirby, & Spradlin, 1988). Pilgrim and Galizio trained conditional discriminations AC, BC, and AD and documented the predicted classes. Reversals of conditional discrimination AD, however, did not produce the expected rearrangement of the classes. Participants responded according to the reversed baseline in tests of symmetry, but not on the other tests. Garotti, de Souza, de Rose, Molina, and Gil (2000) however, replicated the design of Pilgrim and Galizio (1995) and found that 6 out of 9 participants rearranged the classes according to the most recent baseline. A study by Smeets, Barnes-Holmes, Akpinar and Barnes-Holmes (2003) also reversed conditional discriminations and found consistent class rearrangement. These data suggest that classes may in fact rearrange after reversal training and that earlier negative results were due to methodological differences. A subsequent study (Garotti & de Rose, in press) found that classes always rearranged when baseline reviews preceded tests and did not rearrange when, as in the study of Pilgrim and