A behavior-based theory identified 2 characteristics of voluntary acts. The first, extensively explored in operant-conditioning experiments, is that voluntary responses produce the reinforcers that control them. This bidirectional relationship--in which reinforcer depends on response and response on reinforcer--demonstrates the functional nature of the voluntary act. The present article focuses on the second characteristic: a similar bidirectional relationship between reinforcement and the predictability/unpredictability of voluntary acts. Support for the theory comes from 2 areas of research. The first shows that levels of behavioral variability--from highly predictable to randomlike--are directly influenced by reinforcers. Put another way, variability is an operant dimension, analogous to response rate and force. The second source of support comes from psychophysical experiments in which human participants judged the degree to which "choices" by virtual actors on a computer screen appeared to be voluntary. The choices were intermittently reinforced according to concurrently operating schedules. The actors' behaviors appeared to most closely approximate voluntary human choices when response distributions matched reinforcer distributions (an indication of functionality) and when levels of variability, from repetitive to random, changed with reinforcement contingencies. Thus, voluntary acts are characterized by reinforcement-controlled functionality and unpredictability. (Contains 10 figures, 3 tables and 1 footnote.)