Abstract
In four experiments participants made judgements about two possible causes of an effect. The prevalence of the causes was manipulated independently of their degree of contingency with the effect. Significant effects of the prevalence manipulation were obtained: In particular, ratings of the unmanipulated candidate tended to decline as the prevalence of the other candidate increased, and there was also a significant but smaller effect on judgements of the latter. These tendencies were independent of the order in which the two candidates were judged. These results were replicated under two stimulus presentation procedures, the instance list procedure (Experiments 1 and 2) and the trial‐by‐trial procedure (Experiment 3). It was hypothesized that people judge, to some degree, the proportionate strength of the candidates, in other words the proportion of occurrences of the effect in the presence of each, and that the cause prevalence effect is a consequence of this tendency. This hypothesis was supported by the results of Experiment 4: Those participants whose judgements of one candidate were negatively correlated with the frequency of occurrence of the effect in the presence of the other candidate showed a significantly stronger cause prevalence effect than the remainder.