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The goal of the present study was to evaluate the role of verbal stimuli in the production of response variability in humans. College students were distributed into three groups and asked to type three-digit sequences. Participants in the systematic group were instructed to produce sequences according to a rule of their choice; those in the random group were instructed to produce sequences according to chance; and those in the control group were not instructed about how to produce sequences. The experiment employed an ABA design. During the A phases, low-frequent sequences were reinforced (variability contingency), whereas during the B phase, reinforcement was withdrawn (extinction). The results indicated the following: (1) The instructions were efficient at producing systematic and random-like patterns for the systematic and random groups, respectively; in the absence of instructions, a mix of both patterns was observed. (2) Behavior was sensitive to extinction independently of the instructions provided. (3) Systematic patterns favored a more equiprobable distribution of sequences across trials. (4) Reaction times were longer for responding in a systematic than in a random-like fashion. The present findings suggest that individual differences in meeting variability contingencies may be due, at least partially, to instructional control.
