Despite people's strong bias to sort exemplars based on a single dimension, various situations where family resemblance (FR) categories tend to be created have been identified. In a previous study (Ahn 1990b), knowing prototypes or theories underlying categories led subjects to create FR categories. The current study investigates why existence of background knowledge encourages creation of F R categories. Comparison of results from two experiments indicates that there is no intrinsic tie between knowing theories or prototypes and F R structure. The role of background knowledge on FR sorting seems to lie in leading subjects to weight dimensions equally, in helping them to infer unavailable values in favor or F R sorting, and / or in relating surface dimensions in terms of a deeper feature.

Previous studies on category construction have shown that people have a strong bias of creating categories based only on a single dimension. Ahn and Medin (1989) have developed a two-stage model of category construction to explain why we have categories structured on the basis of overall similarity of members in spite of this bias. The current study investigates effects of background knowledge on category construction. The results showed that people created family resemblance categories more frequently when they had a priori knowledge on prototypes of potential family resemblance categories. It was also found that people created family resemblance categories much more frequently when they had knowledge on underlying dimensions which integrated surface features of examples. H o w the two-stage model should be extended is discussed.

Two experiments investigate the conjunction fallacy (judging that conjunctive probabilities are higher than the probabilities of the constituents). The conjunction fallacy was much less for P(E|C) tasks than for P(C|E) tasks. The results are explained in terms of the way people interpret the conditional probabilities. We argue that people prefer to reason from cause to effect (cause-to-effect reasoning heuristic), and for that reason, the instructions given for P(C|E) tasks were misinterpreted, resulting in apparent fallacy. In addition, we provide evidence showing that likelihood judgments are higher with more evidence (more-is-better heuristic).

The features of a concept differ in their centrality. Having a seat is more central to the concept of chair than is having arms. This paper claims that centrality is not a homogeneous phenomenon in that it has at least two aspects, conceptual and naming. We propose that a feature is central to naming in proportion to the feature's category validity, the probability of the feature given the category. In contrast, a feature is conceptually central (immutable) to the extent the feature is depended on by other features. We predict that conceptual and naming centrality diverge as categories become more specific. An experiment is reported that provides corroborating evidence. Increasing the specificity of object categories increased the judged mutability of representative features without affecting their judged appropriateness for determining names.