Proceedings of the Annual Meeting of the Cognitive Science Society

Category formation is constrained by three factors: the perceptual structure of the domain being categorized, the limitations and biases of the learner, and the goals that trigger the learning process in the first place. Many studies of categorization have paid attention to the effects of the structure of the world and some to the biases due to the learner's prior knowledge. This paper explores the third factor: how the goals of the agent at the time of the learning episode affect what categories are formed. In particular it presents an information theoretical account that views categories as a means to increase the agent's chances of achieving its goals. One of the predictions of the theory is that information gain, the average reduction of uncertainty induced by a category, is maximized when the domain is partitioned into about 3 categories, the closest integer to the irrational number e. This prediction is confirmed by evidence derived from anthropological studies of folk classifications of animal and plants by different societies from around the world, and also by an informal observation of the behavior of cognitive scientists. Interestingly, e also emerges from optimization analyses of memory search as well as from experimental work on memory retrieval.