Insight into the structure of conceptual knowledge can be gleaned by examining how statistical regularities in the semantic structure of concepts affect semantic processing. Two similarity judgment experiments revealed that pairs of concepts sharing relatively rare features were judged to be more similar than concepts sharing an equal number of relatively frequent features. Simulations confirmed that these results are consistent with a recurrent connectionist network model of semantic processing in which units corresponding to rare features are activated more quickly and accurately than units corresponding to frequent features. These results support the hypothesis that rare features play a privileged role in the processing of word meanings. © 2009 The Psychonomic Society, Inc.