Human gambling generally involves taking a risk on a low probability high outcome alternative over the more economically optimal high probability low outcome alternative (not gambling). Surprisingly, although optimal foraging theory suggests that animals should be sensitive to the overall probability of reinforcement, the results of many experiments suggest otherwise. For example, they do not prefer an alternative that 100% of the time provides them with a stimulus that always predicts reinforcement over an alternative that provides them with a stimulus that predicts reinforcement 50% of the time. This line of research leads to the conclusion that preference depends on the predictive value of the stimulus that follows and surprisingly, not on its frequency. A similar mechanism likely accounts for the suboptimal choice that humans have to engage in commercial gambling.