Natural categories: Well defined or fuzzy sets?

Thirty college students made category membership decisions for each of 540 candidate exemplar-category name pairs (e.g., apple-fruit ) in each of two separate sessions. For highly typical category members (e.g., chair for the furniture category), and for items unrelated to a category (e.g., cucumber-furniture ), subjects agreed with each other and were consistent in their decisions. However, for intermediate-typicality items (e.g., bookends-furniture ), subjects disagreed with each other and were frequently inconsistent from one session to the next. These data suggest that natural categories are fuzzy sets, with no clear boundaries separating category members from nonmembers.