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In Experiment 1, 6 adults with mild mental retardation were
taught 3 overlapping conditional discriminations in a linear series
structure, establishing the possibility of the emergence of 2
stimulus equivalence classes of 4 stimuli per class. Training
employed balanced trial types in which the discriminative stimuli
were presented in fixed pairs across conditional discriminations
(e.g., 81 and 82, C1 and C2). No participant .showed the
emergence of equivalence classes in initial testing; 1 participant
showed the establishment of classes with repeated testing,
including tests for symmetry alone. In Experiment 2, 6 additional
adults with mild mental retardation were similarly trained, with one
methodological modification: Prior to testing, each comparison
stimulus (e.g. , 81) was presented in some trials with every
stimulus from the opposing class (i .e., A2, 82, C2, and D2). With
this modification, 4 of 6 participants showed the establishment of
equivalence classes during initial testing. The results support
hypotheses about the essential role of simple discrimination
acquisition in equivalence class establishment.