Eleanor Rosch (b. 1938) transformed the psychology of categorization — and, by extension, much of cognitive psychology — by demonstrating that natural categories are not defined by necessary and sufficient features (the classical view inherited from Aristotle) but have graded internal structure organized around prototypical examples. Her prototype theory has influenced research on concepts, language, memory, reasoning, and artificial intelligence.
Prototype Theory
Rosch showed that category membership is a matter of degree, not all-or-none: a robin is a more prototypical bird than a penguin; a chair is more prototypical furniture than a rug. Prototypical members share more features with other category members and fewer features with members of contrasting categories. They are faster to categorize, more likely to be generated as examples, and acquired earlier by children. Categories have fuzzy boundaries — there is no sharp line between cups and bowls, or between red and orange — and typicality effects pervade cognitive processing.
Rosch also discovered that categories are organized into hierarchies with a psychologically privileged "basic level." The basic level (dog, chair, car) is the most inclusive level at which category members share a similar overall shape, can be identified with similar motor programs, and have a single mental image. The basic level is the first level named by children, the level at which objects are most quickly categorized, and the level used in neutral contexts. Superordinate categories (animal, furniture) are too abstract; subordinate categories (golden retriever, rocking chair) are too specific. The basic level represents an optimal balance between informativeness and distinctiveness.