All operant behaviors have multiple characteristics in addition to those criterial for reinforcement, and variation occurs across all. In two experiments humans performed a revealed operant response of 14 or more keystrokes. The first and last were mandated, while the middle 12 (or more) were allowed to vary. There were significant differences in variability among participants, as well as systematic effects of the two different experimental designs. Despite not being reinforced, variability was high. There was systematic topographic bias both for and against letter keys in the center of the keyboard. Furthermore, there were also correlations between measures of variability and of bias.
