Abstract

Humans appear to follow normative rules of inductive reasoning in "premise diversity tasks" that is, they know that dissimilar rather than similar evidence is better for generalising hypotheses. In three experiments, we use a "hypothesis limitation task" to compare a related inductive reasoning skill knowing how to limit hypotheses by using a negative test strategy. Participants are told that one category member has some property (e.g. Dogs have a merocrine gland) and are asked what evidence they would test to ensure that either all (generalisation) or only (limitation) category members have that property (e.g. All/Only mammals have merocrine glands; tests: wolf, bull, crocodile). Despite participants' reluctance to use negative tests in the Wason 2-4-6 task and other reasoning tasks, participants do use normatively correct negative tests in the hypothesis limitation task as often as they use diverse positive tests in the premise diversity task. Moreover, when given a hypothesis limitation task before a rule evaluation task (similar to the 2-4-6 task), the use of negative tests increases. Thus, when testing hypotheses, people can and do use the right kind of test strategy for the task.