Not all exceptions are created equal: Learning of exceptions in pigeons' categorization.

Learning of exceptions - those items that violate a known regularity - takes longer than learning of rule-following items. Studies reporting this disparity have used exceptions that share most of their features with members of the opposite category (crossover exceptions). Yet, exceptions can be distinctly different from members of their own category and other categories as well (oddball exceptions). Here, we trained two groups of pigeons to discriminate two categories containing regular and exception items. For one group, the exceptions were crossovers, whereas for the other, the exceptions were oddballs. Pigeons learned to classify the oddball exceptions faster than the crossover exceptions. Moreover, the regular items were learned more quickly than the crossover exceptions, but more slowly than the oddball exceptions. Deviation from the rule per se is not why exceptions are typically more difficult to learn. Rather, confusability with members of the opposite category hinders learning, whereas distinctiveness facilitates learning.