Long-term stability of equivalence relations in the absence of training or practice.

An adult male with mild mental retardation, whose laboratory history included training in a match-to-sample task on 16 arbitrary relations among 16 visual stimuli and 2 auditory stimuli, had performances that showed the development of 112 additional arbitrary relations that had never been reinforced. In the presented study on 2-year and 3-year follow-up tests for these derived relations, his performances remained stable despite the absence of opportunities to practice these relations between the tests and the absence of explicit feedback during the tests. The similarity to the long-term stability of language was discussed.