It has been repeatedly demonstrated that a robust predictor of the strength of an inductive argument is the similarity between the categories that are the focus of the induction. In this paper we evaluate why similarity is associated with the strength of such arguments. On one view, category similarity makes an argument strong because similarity is partially determined by features that are common to both categories, and the existence of these common features provides reason to think that the

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... clusion is justified. On another view, increased similarity may reflect few differences, so that there are not many reasons to think that the conclusion is unjustified. We evaluate this issue by examining how engagement in inductive reasoning affects the perceived similarity between categories. Our findings suggest that people attempt to find reasons to disbelieve the hypothesis suggested by an argument. They consider differences when evaluating inductions that posit an affirmative contingency, and consider similarities when evaluating inductions that posit a negative contingency. This is done independent of whether the induction is presented in argument form or in the form of a conditional statement, and independent of whether one is evaluating the truth or falsity of the conditional statement.