Hypothesis testing in a rule discovery problem: When a focused procedure is effective

We investigated individuals' ability to use negative evidence in hypothesis testing. We compared performance in two versions of Wason's (1960) rule di scovery problem. In the original version, a triple of numbers- {2, 4, 6}- w as presented as an example of a rule that the experimenter had in mind (i.e ., "increasing numbers''). Participants had to discover the rule by proposi ng new triples. In the other version, the same triple was presented as a co unterexample to the experimenter's rule (i.e., "decreasing numbers''). We p redicted that, in both conditions, participants would form hypotheses based on the features of the triple, and test only instances of the hypothesized rule. However, in the counter-example condition, such focused testing woul d invariably produce negative evidence. As a consequence, participants woul d be forced to revise their hypotheses. The reported results corroborated o ur predictions: Participants solved the counter-example version significant ly better than the original problem.