Tall is typical: Central tendency, ideal dimensions, and graded category structure among tree experts and novices

Many accounts of categorization equate goodness-of-example with central ten dency for common taxonomic categories; the best examples of a category are average members--those that are most similar to most other category members . In the present study, we asked 24 tree experts and 20 novices to rate goo dness-of-example for a sample of 48 trees and found (1) that the internal s tructure of the category tree differed between novices and experts and (2) that central tendency did not determine goodness-of-example ratings for eit her group. For novices, familiarity determined goodness-of-example ratings. For experts, the "ideal" dimensions of height and weediness, rather than a verage similarity to other trees, were the primary predictors of goodness-o f-example ratings for experts. The best examples of tree were not species o f average height, but of extreme height. The worst examples were the weedie st trees. We also found systematic differences in predictors of goodness-of -example as a function of type of expertise. We argue that the internal str ucture of taxonomic categories can be shaped by goal-related experience and is not necessarily a reflection of the attributional structure of the envi ronment. Implications for models of category structure and category learnin g are discussed.