Abstract One enduring principle of rational inference is category inclusion: Categories inherit the properties of their superordinates. In five experiments, I show that people do not consistently apply this principle when evaluating categorical arguments involving natural categories and a single nonexplainable predicate such as all electronic equipment has parts made of germanium, therefore all stereos have parts made of germanium.Participants frequently did not apply the category inclusion rule despite affirming the relevant categorical relation (e.g., stereos are electronic equipment). They failed to apply the rule even when categories were universally quantified unambiguously. Instead, judgments tended to be proportional to the similarity between premise and conclusion categories. Neglect of category inclusion relations was observed using arguments concerning natural kinds, artifacts, and social kinds.