Research Article Dual Processing in Reasoning Two Systems but One Reasoner

ABSTRACT—Human reasoning has been characterized as an interplay between an automatic belief-based systemand a demanding logic-based reasoning system. The present study tested a fundamental claim about the nature of in-dividual differences in reasoning and the processing de-mands of both systems. Participants varying in working memory capacity performed a reasoning task while their executive resources were burdened with a secondary task. Results were consistent with the dual-process claim: The executive burden hampered correct reasoning when the believability of a conclusion conflicted with its logical va-lidity, but not when beliefs cued the correct response. However, although participants with high working mem-ory spans performed better than those with lower spans in cases of a conflict, all reasoners showed similar effects of load. The findings support the idea that there are two reasoning systems with differential processing demands, but constitute evidence against qualitative individual dif-ferences in the human reasoning machinery. Human reasoners are prone to various errors in logical reasoning and judgment. No matter how small the probability of a new terrorist attack on the 11th day of the ninth month of the year may be, for example, many people will nevertheless refrain from flying on that specific day. They believe that taking the car is the safer option. However, the resulting increased risk of dying in a fatal traffic accident may actually outweigh the risks associated with flying (e.g., Gigerenzer, 2004). People’s objective risk judgment is thus biased by the vividness of their recollection of the collapsing twin towers. This common human tendency to base judgments on prior beliefs and intuition rather than on a logical reasoning process biases performance in many classic reasoning tasks (Evans, 2002; Kahneman, Slovic, & Tversky, 1982). Influential dual-process theories of thinking have explained people’s ‘‘rational thinking failure’ ’ by positing two different hu-man reasoning systems (e.g., Epstein, 1994; Evans, 2003; Evan