A subject's performance under a conditional-discrimination procedure defines conditional relations between stimuli: "If Al, then Bl; if A2, then B2." The procedure may also generate matching to sample. If so, the stimuli will be related not only by conditionality, but by equivalence: Al and Bl will become equivalent members of one stimulus class, A2 and B2 of another. One paradigm for testing whether a conditional-discrimination procedure has generated equivalence relations uses three sets of stimuli, A, B, and C, three stimuli per set.Subjects learn to select Set-B and Set-C colnparisons conditionally upon Set-A samples. Having been explicitly taught six sample-colniparison relations, AlBl, AlCl, A2B2, A2C2, A3BM, and A3C3, subjects prove imnmediately capable of matching the B-and C-stimuli; six new relations emerge (BlCl, B2C2, B3C3, CIBl, C2B2, CMB3). The 12 stimulus relations, six taught and six emergent, define the existence of three three-member stimulus classes, AIBlCl, A2B2C2, and A3B3C3. This paradigm was expanded by introducing three more stimuli (Set D), and teaching eight children not only the AB and AC relations but DC relations also-selecting Set-C comparisons conditionally upon Set-D samples. Six of the children proved immediately capable of matching the B-and D-stimuli to each other. By selecting appropriate Set-B comparisons conditionally upon Set-D samples, and Set-D comparisons conditionally upon Set-B samples, they demonstrated the existence of three four-member stimulus classes, AIBlClDl, A2B2C2D2, and A3B3C3D3. These larger classes were confirmed by the subjects' success with the prerequisite lower-level conditional relations; they were also able to select Set-D comparisons conditionally upon samples from Sets A and C, and to do the BC and CB matching that defined the original three-member classes. Adding the three DC relations therefore generated 12 more, three each in BD, DB, AD, and CD. Enlarging each class by one member brought about a disproportionate increase in the number of emergent relations. Ancillary oral naming tests suggested that the subject's application of the same name to each stimulus was neither necessary nor sufficient to establish classes of equivalent stimuli.Key words: conditional discrimination, matching to sample, stimulus equivalence, stimulus classes, stimulus control, key press, children Given two discriminative stimuli, Bl and B2, a

Where do equivalence relations come from? One possible answer is that they arise directly from the reinforcement contingency. That is to say, a reinforcement contingency produces two types of outcome: (a) 2-, 3-, 4-, 5-, or n-term units of analysis that are known, respectively, as operant reinforcement, simple discrimination, conditional discrimination, second-order conditional discrimination, and so on; and (b) equivalence relations that consist of ordered pairs of all positive elements that participate in the contingency. This conception of the origin of equivalence relations leads to a number of new and verifiable ways of conceptualizing equivalence relations and, more generally, the stimulus control of operant behavior. The theory is also capable of experimental disproof.

A retarded boy, unable to read printed words orally or with comprehension, could match spoken words to pictures and could name pictures. After being taught to match spoken to printed words, he was then capable of reading comprehension (matching the printed words to pictures) and oral reading (naming the printed words aloud).

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