This thesis was designed to investigate the effect of task complexity upon how humans estimate duration. Previous task complexity research suggests that duration is overestimated with simple tasks and underestimated with complex tasks. One-hundred and forty-two first and second year university students participated. Twelve experiments were conducted, which required participants to complete computer generated jigsaw puzzles and periodically estimate how long they thought they had been doing the puzzle. In Experiment 1, participants were required to complete a jigsaw puzzle before making an estimate. In the remaining eleven experiments, estimates were made throughout the session whilst participants worked on the jigsaw puzzle. In the first four experiments, a task was complex if there were more puzzle pieces and simpler if there were fewer puzzle pieces. There were no significant results obtained from the first four experiments. Given the lack of effect from the first four experiments, the next two experiments partially replicated two task complexity studies to determine how task complexity can be used as an explanation for why estimations of duration differ. Again, there were no significant results obtained from these two experiments. The next four experiments tested whether people's estimates of duration were affected by the rate of reinforcement they receive (i.e., successfully moving a puzzle piece to a new location per unit time). In the first of these two experiments (7 and 8) there was no effect of the manipulation, which consisted of decreasing the distance which a puzzle piece could be moved on the screen, relative to the distance the computer mouse was moved and fixing the speed at which a puzzle piece could be moved. In Experiments 9 and 10, more discriminative stimuli were used to indicate to participants that a change in the reinforcement rate was occurring. There was a significant result in Experiment 9 in one condition but this effect was not replicated in Experiment 10. In Experiment 11, the reinforcement rate was reduced to zero and there was a significant effect on participants' estimates of duration. However, these results suggested a confound between whether the reinforcement rate or not being able to access the jigsaw puzzle was affecting estimates of duration. In Experiment 12, access to the jigsaw puzzle was limited, whilst simultaneously controlling the reinforcement rate and the results showed that not having access to the jigsaw puzzle affected how participants estimate duration. These findings suggest that information can act as reinforcement, enabling a person to engage in private behaviour. When there is no access to reinforcement, time 'drags' for humans.