Humans prefer (conditioned) rewards that follow greater effort (Aronson & Mills, 1959). This phenomenon can be interpreted as evidence for cognitive dissonance (or as justification of effort) but may also result from (1) the contrast between the relatively greater effort and the signal for reinforcement or (2) the delay reduction signaled by the conditioned reinforcer. In the present study, we examined the effect of prior force and prior time to produce stimuli associated with equal reinforcement. As expected, pressing with greater force or for a longer time was less preferred than pressing with less force or for a shorter time. However, participants preferred the conditioned reinforcer that followed greater force and more time. Furthermore, participants preferred a long duration with no force requirement over a shorter duration with a high force requirement and, consistent with the contrast account but not with the delay reduction account, they preferred the conditioned stimulus that followed the less preferred, shorter duration, high-force event. Within-trial contrast provides a more parsimonious account than justification of effort, and a more complete account than delay reduction.