Abstract

Experimental evidence on reasoning and decision making has been used to argue both that human rationality is adequate and that it is defective. The idea that reasoning involves not one but two mental systems (see Evans and Over, 1996; Sloman, 1996; Stanovich, 2004 for reasoning, and Kahneman and Frederick, 2005 for decision making) makes better sense of this evidence. ‘System 1’ reasoning is fast, automatic, and mostly unconscious; it relies on ‘fast and frugal’ heuristics (to use Gigerenzer’s expression (Gigerenzer et al., 1999)) offering seemingly effortless conclusions that are generally appropriate in most settings, but may be faulty, for instance in experimental situations devised to test the limits of human reasoning abilities. ‘System 2’ reasoning is slow, consciously controlled and effortful, but makes it possible to follow normative rules and to overcome the shortcomings of system 1 (Evans and Over, 1996). The occurrence of both sound and unsound inferences in reasoning experiments and more generally in everyday human thinking can be explained by the roles played by these two kinds of processes. Depending on the problem, the context, and the person (the ability for system 2 reasoning is usually seen as varying widely between individuals, see Stanovich and West (2000)) either system 1 or system 2 reasoning is more likely to be activated, with different consequences for people’s ability to reach the normatively correct solution (Evans, 2006). The two systems can even compete: system 1 suggests an intuitively appealing response while system 2 tries to inhibit this response and to impose its own norm-guided one. Much evidence has accumulated in favour of such a dual view of reasoning (Evans, 2003, in press; for arguments against, see Osman, 2004). There is, however, some vagueness in the way the two systems are characterized. Instead of a principled distinction, we are presented with a bundle of contrasting features—slow/fast, automatic/controlled, explicit/implicit, associationist/rule based, modular/central— which, depending on the specific dual process theory, are attributed more or less exclusively to one of the two systems..