The Prevalence Effect is Determined by Past Experience, Not Future Prospects

In a laboratory task similar to an X-ray baggage search at an airport, Wolfe, Horowitz, and Kenner (2005) reported a "prevalence effect" (i.e., a very high miss rate) when the presence of a target is very infrequent. The present study tested whether this prevalence effect is the result of a voluntary top-down control for future prospect or an implicit bottom-up priming from past experience. Experiments 1 and 2 showed that, regardless of instructions given on the likelihood of target presence, the magnitude of prevalence (i.e., the miss rate) was determined only by the actual prevalence of the target. In Experiments 3 and 4, target prevalence was indicated by background color on a trial-by-trial basis. Some blocks (i.e., constant blocks) were either comprised of all high-prevalence trials or all low-prevalence trials, whereas in other blocks (i.e., mixed blocks) high-prevalence and low-prevalence trials were randomly mixed. Target prevalence significantly affected the miss rate in the constant blocks, but had no effect in the mixed blocks. Overall, the prevalence effect is essentially the result of past experience and is not affected by future prospect.