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TRENDS in Cognitive Sciences

Vol.7 No.10 October 2003

In two minds: dual-process accounts of reasoning
Jonathan St. B.T. Evans
Centre for Thinking and Language, University of Plymouth, Plymouth, PL4 8AA, UK

Researchers in thinking and reasoning have proposed recently that there are two distinct cognitive systems underlying reasoning. System 1 is old in evolutionaryterms and shared with other animals: it comprises a set of autonomous subsystems that include both innate input modules and domain-specific knowledge acquired by a domain-general learning mechanism. System 2 is evolutionarily recent and distinctively human: it permits abstract reasoning and hypothetical thinking, but is constrained by working memory capacity and correlated with measures of generalintelligence. These theories essentially posit two minds in one brain with a range of experimental psychological evidence showing that the two systems compete for control of our inferences and actions. The idea that there are two distinct kinds of reasoning has been around for as long as philosophers and psychologists have written about the nature of human thought. However, it is only in recentyears that cognitive scientists have proposed the striking and strong claim that there are two quite separate cognitive systems underlying thinking and reasoning with distinct evolutionary histories. These two systems are sometimes described as Implicit and Explicit [1,2] although some dual-process theorists prefer to emphasize the functional differences between the two systems and leave open therelation to consciousness [3,4]. In this article, I will therefore use the neutral terms System 1 and System 2 as introduced by Stanovich and West [5,6]. Contemporary interest in the dual-process accounts of reasoning is evidenced by the wider application to related fields such as judgment and decision making [7] and exciting developments in neuropsychological studies of reasoning, described below.System 1 is generally described as a form of universal cognition shared between humans and animals. It is actually not really a single system, but a set of sub-systems that operate with some autonomy [8,9]. System 1 includes instinctive behaviours that are innately programmed, and would include any innate input modules of the kind proposed by Fodor [10] which are not be confused with morequestionable [11] recent claims for domain-encapsulated innate modules that control specific behaviours. The System 1 processes that are most often described, however, are those that are formed by associative learning processes of the kind produced by neural networks [12]. The
Corresponding author: Jonathan St. B.T. Evans (jevans@plymouth.ac.uk).

autonomy of such systems reflects the domain-specificnature of the learning, even though the learning mechanism itself is domain-general [13]. Dual-process theorists generally agree that System 1 processes are rapid, parallel and automatic in nature: only their final product is posted in consciousness. There is at least one contemporary research programme in which researchers are attempting to account for all reasoning results in terms of System 1level processes [14]. However, I shall provide substantial evidence that postulation of a second system is required. System 2 is believed to have evolved much more recently and is thought by most theorists to be uniquely human. System 2 thinking is slow and sequential in nature and makes use of the central working memory system that has been so intensively studied in the psychology of memory[15,16]. Despite its limited capacity and slower speed of operation, System 2 permits abstract hypothetical thinking that cannot be achieved by System 1. Consider the case of decision-making. We might (and frequently do) decide our actions on the basis of past experience, doing what has worked well in the past. Such intuitive decisions require little reflection. However, we can also make decisions by...