Natural sources of internal category structure: Typicality,familiarity, and similarity of birds

Typicality ratings of 53 types of birds by University of California, Berkeley undergraduates (Rosch, 1975b) were more strongly correlated with the number of related species than with the frequency of the birds in the observers’ immediate environment or with the frequency of mention of the birds in written materials. Furthermore, the subjects made finer discriminations among the most typical birds (passerines) than among the less typical birds (nonpasserines), yet disagreed more often in identifying passerines and reported greater difficulty in judging the similarities among passerines. A single model explains both sets of results: passerines appear to be densely and continuously spread through the bird similarity space, whereas nonpasserines are more sparsely and discontinuously distributed, leading to the choice of passerines as both more typical and more difficult to categorize than nonpasserines.