Complex reasoning, such as that elicited by GRE 'logical' reasoning problems, is demanding for human reasoners and beyond the competence of any existing computer program. We report four experiments carried out to investigate the question of what makes these problems difficult. The experiments established three causes of difficulty: the nature of the logical task (Experiment 1), the nature of the incorrect foils (Experiment 2), and the nature of the correct conclusions (Experiments 3 and 4).

We report two experiments on the effects of diagrams on reasoning. Both studies used "double disjunctions", e.g.: Raphael is in Tacoma or Julia is in Atlanta, or both. Julia is in Atlanta or Paul is in Philadelphia, or both. What follows? Subjects find it difficult to deduce a valid conclusion, such as: Julia is in Atlanta,