When do base rates affect predictions?

Recent studies have shown that when people make predictions, they often neglect base-rate considerations. Instead of considering what typically happens in situations like the one being judged, they rely on the extent to which the judged case is representative of the possible prediction categories--although if representativeness fails to provide a clear guide to prediction, people will resort to base-rate considerations. Manis, Dovalina, Avis, and Cardoze (1980) have recently argued against this conclusion, presenting as evidence a series of experiments in which subjects predicted the category membership of individuals depicted in each of a set of photos. They found that base rates had a clear effect on discrete predictions (i.e., a majority of the photos was predicted as belonging to the larger category), and a smaller effect on the confidence subjects attached to those predictions. However, only a minority of these photos could be readily classified by representativeness. As a result, Manis et al.'s findings can be reinterpreted in a way that makes them compatible with previous findings. In this light, their study emerges as a constructive replication of earlier results demonstrating judgment by representativeness. (Author)