Abstract Questions about how to improve human judgment and reasoning are of theoretical and practical interest, notwithstanding the continuing controversy over whether people are “rational”. Improving judgment may involve modifying people's processes to fit their environments better, or vice versa. We illustrate the latter approach in a study of diagnostic reasoning in which subjects learned to distinguish two fictitious diseases. Prior findings suggest that people may judge the likelihood of a diagnostic category on the presence or absence of features that are typical of, rather than diagnostic of, the category. We varied the structure of the information provided to subjects without attempting to modify their judgmental processes. In an “independent” format, subjects learned about each disease separately; in a “contrastive” format, information about the two diseases was juxtaposed to highlight distinctive features. Subjects in the two conditions formed different disease concepts. Diagnoses following contrastive training were much closer to the statistically prescribed judgments based on likelihood ratios. Interventions that modify the environment may provide an alternative approach where it is difficult to modify people's processes. Effective design of such interventions is one motivation for directing research toward understanding how task characteristics affect the use of and the outcomes of judgment and reasoning processes.