Individuals often estimate the probability of future events by the ease with which they can recall or cognitively construct relevant instances. Previous research has not precisely identified the cognitive processes mediating this "availability heuristic." Two potential mediators (imagery of the event, perceived reasons or causes for the event) were examined using the 1984 presidential debate as the to-be-predicted event. College students (N=205) were randomly assigned to one of six conditions in a 2x3 factorial design. The first factor varied who won the debate, Reagan or Mondale. The second factor varied the cognitive availability of the debate outcome and reasons for the outcome. Subjects read scenarios which either described the debate and outcome (imagine condition), included plausible reasons for the outcome (reason condition), or a combination of both scenarios (imagine and reason condition). Subjects then answered questions on politics, the media, candidate preference, and an estimate of who would win the actual debate. The results indicated that debate predictions were affected by participants' candidate preferences and self-generated reasons. In addition, debate predictions were affected by the availability of reasons but not by imagery of the outcome. Simply imagining a winner had no effects on predictions, but considering reasons for the outcome did significantly affect predictions. These findings are consistent with research indicating the importance of causal thinking in judgment under uncertainty. (NRB)