The Easy Path From Many To Much: the Numerosity Heuristic

Abstract People are especially sensitive to numerosity as a cue for judging quantity or probability. That is, people sometimes judge amount or likelihood on the basis of the number of units into which a stimulus is divided without fully considering other important variables (e.g., the size of the units). People appear to be especially likely to make use of this "numerosity heuristic" when their cognitive resources are taxed. Consistent with this idea, five experiments showed that people are especially likely to overinfer quantity or probability from numerosity (a) when they are asked to make inherently difficult judgments, (b) when they are asked to render judgments while performing a concurrent task, and (c) when they are forced to make especially rapid judgments. In addition to their implications for the numerosity heuristic, the broad implications of these findings for the study of judgment are discussed.