Similarity, confusability, and the density hypothesis

Results from several letter- and digit-identification studies have been interpreted (Appelman & Mayzner, 1982; Krumhansl, 1982) as providing support for the hypothesis that psychological similarity is influenced by the local density of items in the stimulus space. This conclusion is questioned on the grounds that density was not directly manipulated in the studies, thus alternative explanations based on other stimulus characteristics cannot be excluded. In the present article six experiments are reported in which stimulus density was manipulated. In three experiments using similarity ratings and two using discrimination confusions, no effect of stimulus density was found. However, the identification counterpart of one of the discrimination studies did provide evidence of an effect of density on response probabilities. It is concluded that stimulus density can affect identification performance through its influence on the choice process implicit in any identification task, but is not an important determinant of psychological similarity.