Evans Dual Processing 2008

ANRV331-PS59-10 ARI 4 November 2007 20:23

Dual-Processing Accountsof Reasoning, Judgment,and Social CognitionJonathan St. B. T. EvansCenter for Thinking and Language, School of Psychology, University of Plymouth,Plymouth PL4 8AA, United Kingdom; email: [email protected]

Annu. Rev. Psychol. 2008. 59:25578

First published online as a Review in Advance onMay 31, 2007

The Annual Review of Psychology is online athttp://psych.annualreviews.org

This articles doi:10.1146/annurev.psych.59.103006.093629

Copyright c 2008 by Annual Reviews.All rights reserved

0066-4308/08/0203-0255$20.00

Key Words

thinking, reasoning, decision-making, social cognition,dual-process theory

AbstractThis article reviews a diverse set of proposals for dual processingin higher cognition within largely disconnected literatures in cog-nitive and social psychology. All these theories have in common thedistinction between cognitive processes that are fast, automatic, andunconscious and those that are slow, deliberative, and conscious. Anumber of authors have recently suggested that there may be twoarchitecturally (and evolutionarily) distinct cognitive systems under-lying these dual-process accounts. However, it emerges that (a) thereare multiple kinds of implicit processes described by different the-orists and (b) not all of the proposed attributes of the two kinds ofprocessing can be sensibly mapped on to two systems as currentlyconceived. It is suggested that while some dual-process theories areconcerned with parallel competing processes involving explicit andimplicit knowledge systems, others are concerned with the inuenceof preconscious processes that contextualize and shape deliberativereasoning and decision-making.

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Contents

INTRODUCTION. . . . . . . . . . . . . . . . . 256FEATURES ATTRIBUTED

TO DUAL PROCESSESAND SYSTEMS . . . . . . . . . . . . . . . . . 258Consciousness . . . . . . . . . . . . . . . . . . . . 258Age of Evolution . . . . . . . . . . . . . . . . . 259Functional Characteristics . . . . . . . . 261Individual Differences . . . . . . . . . . . . 262

APPLICATIONS OFDUAL-PROCESS THEORIESIN DIFFERENT DOMAINS . . . . 263Dual-Process Theories of

Reasoning . . . . . . . . . . . . . . . . . . . . 263Dual-Process Theories of

Judgment and DecisionMaking . . . . . . . . . . . . . . . . . . . . . . . 265

Dual-Process Theories of SocialCognition . . . . . . . . . . . . . . . . . . . . . 268

CONCLUSIONS. . . . . . . . . . . . . . . . . . . 270

INTRODUCTION

Dual-processing accounts of human behaviorabound in cognitive and social psychology. Somany authors have appealed to dual processesin so many different ways that it has proveda complex and challenging task to draw to-gether any coherent overview of this topic.The review is restricted to higher cogni-tive processes typically described as thinking,reasoning, decision-making, and social judg-ment, although selected work from outside ofthese domains is also included where directlyrelevant. For example, there is some refer-ence to work in the psychology of learning andmemory, the philosophy of mind, and evolu-tionary psychology.

What dual-process theories have in com-mon is the idea that there are two differ-ent modes of processing, for which I use themost neutral terms available in the literature,System 1 and System 2 processes (Kahneman& Frederick 2002, Stanovich 1999). Almostall authors agree on a distinction between

processes that are unconscious, rapid, auto-matic, and high capacity, and those that areconscious, slow, and deliberative. Differentauthors have proposed a number of namesfor the two kinds of thinking they contrast,some of which are shown in Table 1. The la-bels are aligned in a manner consistent withgeneric dual-system theory (see below) un-der the headings System 1 and System 2. De-spite the tidy way in which all these labels lineup, readers should beware of inferring thatthere are necessarily just two systems or justtwo kinds of dual-processing theory for rea-sons that will become clear later. Some au-thors propose only dual-process distinctionswithout assumptions about underlying cogni-tive systems; some propose parallel and somesequential relationships between the two pro-cesses, and so on.

Attempts have been made to map vari-ous dual-process accounts into a generic dual-system theory (Evans 2003; Evans & Over1996; Stanovich 1999, 2004). A major issuefor this review is to consider whether sucha grand theory is sustainable, or whether, inspite of rst appearances, we need to classifydual-process theories as being of different anddistinct kinds. I approach this question in partby considering the clusters of attributes sup-posedly belonging to System 1 and 2, whichhave been extracted from the numerous dual-process theories to be found in the literature.Of course, not all authors have made explicitstatements about all of these attributes, butwhen they do so, they tend to make commentsthat are remarkably consistent from theory totheory. These putative features of Systems 1and 2 are somewhat arbitrarily grouped hereinto four categories (see Table 2) and each isdiscussed in turn prior to the review of specicdual-process theories of higher cognition.

It should be noted that the attributes listedin Table 2 do not include emotion, the dis-cussion of which is generally beyond the scopeof this review. Although many authors ig-nore emotion altogether in the elds reviewedhere, it is clear that emotional processingwould be placed in the System 1 rather than

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Table 1 Labels attached to dual-processes in the literature, aligned on theassumption of a generic dual-system theory

References System 1 System 2Fodor (1983, 2001) Input modules Higher cognitionSchneider & Schiffrin (1977) Automatic ControlledEpstein (1994), Epstein & Pacini (1999) Experiential RationalChaiken (1980), Chen & Chaiken (1999) Heuristic SystematicReber (1993), Evans & Over (1996) Implicit/tacit ExplicitEvans (1989, 2006) Heuristic AnalyticSloman (1996), Smith & DeCoster (2000) Associative Rule basedHammond (1996) Intuitive AnalyticStanovich (1999, 2004) System 1 (TASS) System 2 (Analytic)Nisbett et al. (2001) Holistic AnalyticWilson (2002) Adaptive unconscious ConsciousLieberman (2003) Reexive ReectiveToates (2006) Stimulus bound Higher orderStrack & Deustch (2004) Impulsive Reective

Table 2 Clusters of attributes associated with dual systems of thinking

System 1 System 2Cluster 1 (Consciousness)Unconscious (preconscious) ConsciousImplicit ExplicitAutomatic ControlledLow effort High effortRapid SlowHigh capacity Low capacityDefault process InhibitoryHolistic, perceptual Analytic, reectiveCluster 2 (Evolution)Evolutionarily old Evolutionarily recentEvolutionary rationality Individual rationalityShared with animals Uniquely humanNonverbal Linked to languageModular cognition Fluid intelligenceCluster 3 (Functional characteristics)Associative Rule basedDomain specic Domain generalContextualized AbstractPragmatic LogicalParallel SequentialStereotypical EgalitarianCluster 4 (Individual differences)Universal HeritableIndependent of general intelligence Linked to general intelligenceIndependent of working memory Limited by working memory capacity

www.annualreviews.org Dual Processing in Higher Cognition 257

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System 2 list. Emotion is explicitly linked tothe rst system in some dual-process accounts(Epstein 1994), implied by the neurologicalregions identied in others (Lieberman 2003),and specically attributed to automatic pro-cesses in some recent accounts of social cog-nition (Hassin et al. 2005). Moreover, the ideathat emotions play a key role in decision mak-ing is being developed in some contemporarywork that contrasts a fast emotional basis fordecision making with a slower and more delib-erative cognitive basis (Haidt 2001, Hanoch &Vitouch 2004, Wang 2006).

FEATURES ATTRIBUTEDTO DUAL PROCESSESAND SYSTEMS

Consciousness

A central concern for philosophers of mind,the problem of consciousness is one that hasengendered a resurgence of interest in re-cent years in psychology and neuroscience(Velmans 2000). However, the concept of con-sciousness has had a somewhat checkered his-tory in psychology. Mentalism and introspec-tionism dominated philosophy of mind andearly laboratory studies of the nineteenth cen-tury when psychology rst emerged as a dis-tinct discipline. Thereafter, a series of move-ments led instead to an emphasis on whatis automatic and unconscious in the controlof human behavior. These are sometimes de-scribed as the psychoanalytic, behavioral, andcognitive unconscious (Uleman 2005). Eachrefers to a movement that undermined theconcept of consciousness and the explanation(in scientic psychology) of behavior in termsof conscious beliefs and desires. Freud andother psychoanalysts introduced the world tothe notion of an unconscious mind motivat-ing our behavior with a combination of innatedrives and repressed emotions as well as a con-scious mind prone to rationalization and self-deception. Behaviorists such as Watson, Hull,and Skinner demonstrated associative and in-strumental learning processes in both humans

and animals in theories that apparently al-lowed no place for consciousness and mental-istic accounts of behavior. However, the emer-gence of cognitive science and the widespreadacceptance of a computational theory of mindled in the later part of the twentieth century tothe idea of a cognitive unconscious (Kihlstrom1987, Reber 1993) in the form of complex in-formation processing in the brain that is con-ducted without conscious awareness. Someauthors are now advocating a new uncon-scious that also incorporates motivation andemotion (Hassin et al. 2005).

While the problem of what conscious-ness is may seem intractable, the study of itsfunction and evolution seems more promis-ing. Dual-process theories of cognition effec-tively address the important question of thecognitive correlates of consciousness. The at-tributes listed for Systems 1 and 2 under Clus-ter 1 (Table 2) all reect the proposed dif-ferences between the cognitive unconsciousand the cognitive conscious. Authors talkof processes that are explicit and controlled(rather than implicit and automatic) reect-ing two key concepts of consciousness: aware-ness and volition. Consciousness is also in-herently slow, sequential, and capacity lim-ited. This might be taken to mean that folkpsychological accounts of mind are adequateat the System 2 level provided the System 1level of mind is also recognized (Frankish2004). In other words, System 2 is a formof thinking under intentional level con-trol, supported by unconscious processes inSystem 1 that deliver percepts, memories,and so on. However, this happy state of af-fairs, which leaves us in control of ourbehavior, is contradicted by much psycho-logical research. Many researchers have em-phasized the fact that unconscious processesmay control our behavior without us be-ing aware of them doing so, and that con-scious reasoning in System 2 is often usedfor the confabulation of explanations forthese behaviors (Evans & Over 1996, Nis-bett & Wilson 1977, Stanovich 2004, Wason& Evans 1975, Wilson 2002, Wilson &

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Dunn 2004). It appears that we use theconcepts of belief-desire psychology to the-orize about our behavior as well as that ofothers. To muddy the waters further, some so-cial psychologists are claiming evidence of in-tentionality in unconscious processing (Barghet al. 2001, Bargh & Ferguson 2000).

The distinction between automatic andcontrolled cognition has been extensively re-searched in the study of lower-order cogni-tion such as that involved in perception, at-tention, and the acquisition of motor skills, ina tradition dating from Schneider & Shiffrin(1977) and more recently linked with neuro-science (Monsell & Driver 2000). In this tradi-tion, automatic processes are typically viewedas having been automated from those thatwere once controlled and conscious, an ap-proach that seems to have had a strong inu-ence on the development of dual-process ac-counts of social cognition (Chaiken & Trope1999). However, as Sloman (2002) points out,this conception is too limited for the pur-poses of dual-process accounts of higher cog-nition, in which we may feel an experience ofconict between an intuition acquired fromexperience and a conscious piece of reason-ing. Moreover, there is a contrasting liter-ature on implicit learning (Berry & Dienes1993, Reber 1993, Sun et al. 2005) that pro-vides much evidence that people can acquireimplicit knowledge, for example to predictor control a complex system, without everknowing an explicit rule that they could state.Hence, the term automatic is used here sim-ply as a contrastive with controlled, imply-ing no assumption about how such processeswere acquired.

An operational denition of conscious-ness that seems to have appeared (oftenimplicitly) in dual-process theories is thatSystem 2 thinking requires access to a cen-tral working memory system of limited ca-pacity, whereas System 1 does not. What weare aware of at a given time is representedin this working memory, through which con-scious thinking ows in a sequential manner.This would seem to be a working memory of

the kind originally described by Baddeley &Hitch (1974), which has not only short-termmemory capacity but also executive and in-hibitory functions, a theory that has engen-dered a huge research literature in its ownright (see Gathercole 2003). (Consciousnessis also closely associated with working mem-ory in global workspace theorysee Baars &Franklin 2003.) The association of consciousthought with such a working memory explainsthe slow, sequential, and low-capacity natureof System 2 as well as its relation to individ-ual differences in cognitive ability (see below).However, skeptics may see this as the only rmfoundation on which the various dual-processtheories stand: There is one conscious work-ing memory system and everything else.

Age of Evolution

The idea that System 1 cognition evolved ear-lier than System 2 is a recurring theme indual-process theories (Evans & Over 1996,Epstein & Pacini 1999, Reber 1993, Stanovich1999). System 2 is thought to be associatedwith language, reective consciousness, andhigher-order control and with the capacity tothink hypothetically about future and coun-terfactual possibilities. Authors have often as-serted that such characteristics of thought aredistinctively or uniquely human, while at thesame time arguing that System 1 cognition,shared with other animals, continues to con-trol much of our behavior. However, thereis considerable evidence of a distinction be-tween stimulus-bound and higher-order con-trol process in many higher animals (Toates2006), including rodents, which could be seenas the biological foundations for the System 1and 2 cognition in humans. There is also ev-idence that primates, especially chimpanzees,have the capacity for higher-order mental rep-resentations, manifest as rudimentary theoryof mind (Mithen 1996, Whiten 2000), albeitvery limited in comparison with the ability ofthe human mind for meta-representation andhigher-order intentionality.

The proposal that System 1 cognitionis evolutionarily old and shared with other


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animals is also problematic because it is al-most certainly not one system with a singleevolutionary history. There are a number ofdistinct possible types of implicit cognitiveprocesses, including forms of learning, auto-maticity, modular cognition, and the prag-matic processes that have been particularlystudied in dual-process theories of deduc-tive reasoning (see below). The concept ofcognitive modules was introduced by Fodor(1983) in a dual-process theory that distin-guished between input modules (such as thoseinvolved in vision and language) and gen-eral purpose, central cognition. Fodor pro-posed a number of strict criteria for mod-ules including the requirements that theyare innate, domain-specic, have specializedand isolated databases (knowledge encap-sulation), are neurologically localized, andare associated with specic disorders. Someschools of evolutionary psychology later ar-gued that the mind should consist mostly orentirely of domain-specic cognitive mod-ules, even when engaged in higher orderreasoning (Cosmides & Tooby 1992, 1996;Pinker 1997; Samuels 1998; Sperber 1994;Tooby & Cosmides 1992). This view includedstrong claims that domain-general processeswould have little part to play in human cog-nition, such as that of Tooby & Cosmides(1992, p.112): . . . there is a host of . . . rea-sons why content-free, general-purpose sys-tems could not evolve, could not manage theirown reproduction, and would be grossly in-efcient and easily outcompeted if they did.Not surprisingly, this massive modularity hy-pothesis has been strongly attacked by dual-process theorists (Fodor 2001, Over 2003,Stanovich 2004, Stanovich & West 2003).More recent writings of evolutionary psy-chologists appear to have more compatibil-ity with dual-processing frameworks, how-ever, as they acknowledge the extraordinaryand distinctive features of human higher cog-nition (Cosmides & Tooby 2000, Sperber2000). There has also been a recent trendto weaken the criteria for modules to makemassive modularity a more credible hypoth-

esis (Barrett & Kurzban 2006, Carruthers2006).

It seems unsustainable to argue that thereis just one form of implicit processing, inSystem 1, all of which is evolutionarily oldand shared with other animals. For example,we may have forms of modular cognition thatare relatively old (e.g., vision, attention) andothers that are much more recent and distinc-tively human (e.g., language, theory of mind).Conditioning and other forms of associativelearning appear to be ancient and shared withother animals, but forms of explicit memory,and in particular the human belief system,seem to be much more recent. However, al-though the notion that there are distinct im-plicit and explicit memory systems is centralto a number of the dual-process theories thatare considered in this review, it could wellbe an error to think of the latter as uniquelyhuman in origin. There are powerful evolu-tionary arguments (as well as neurological ev-idence) for multiple systems of learning andmemory in both humans and other animals(Carruthers 2006, Sherry & Schacter 1987).For example, Sherry & Schacter (1987), whointerestingly referred to System 1 and 2 mem-ory, noted that . . . a strong case can be madefor a distinction between a memory systemthat supports gradual or incremental learn-ing and is involved in the acquisition of habitsand skills and a system that supports rapidone-trial learning and is necessary for formingmemories that represent specic situationsand episodes (p. 446). Taken in conjunc-tion with the evidence of higher-order con-trol systems in animals (Toates 2006), thesearguments suggest that dual-system theoristswould be better off claiming that System 2cognition is uniquely developed, rather thanuniquely present, in modern humans. Such anargument also has much greater evolutionaryplausibility.

If Systems 1 and 2 incorporate differentmemory as well as reasoning systems, then itmay be a mistake to assume that any inu-ence of prior knowledge on reasoning neces-sarily arises in System 1. For example, Goel

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(2005) has questioned the idea that the be-lief bias in reasoning that theorists have as-sociated with System 1 processing (Evans &Over 1996, Stanovich 1999) could be ancientin origin or shared with animals that lack anexplicit belief system. In support of this, hehas evidence from neural imaging studies thatbelief bias arises in the prefrontal cortex, anarea most strongly developed in the modernhuman brain (Goel et al. 2000, Goel & Dolan2003). However, a dual-system theorist canreply that although System 1 has much incommon with animal cognition, it looks verydifferent in a brain that has System 2. ThusStanovich (2004, Chapter 2), for example,suggests that goals that are acquired reec-tively through System 2 can, through repeatedactivation, be installed into rigid implicit pro-cessing mechanismsa kind of automation ofthought. This may be why Stanovich (2004)now prefers to talk about TASSthe set of au-tonomous subsystemsrather than System 1.Certainly, there seems to be little foundationin arguments based on age of evolution forthe claim that all implicit processes belongin any useful senseto a single system.

Functional Characteristics

Along with already discussed attributes ofSystem 1 as rapid and automatic and System 2as slow and controlled go a number of func-tional differences attributed to the two kindsof cognition, as shown in Table 2 underCluster 3. It appears that conscious thoughtis inherently sequential, whereas many theo-rists suppose the rapid processing and highcapacity of System 1 reects use of paral-lel processes. System 1 has been character-ized as associative by Sloman (1996, 2002;see also Smith & DeCoster 2000), as con-trasted with rule-based cognition in System2. Those authors who are not focused onthe idea of innate modules certainly empha-size the experiential nature of System 1 (forexample, Epstein & Pacini 1999, Evans &Over 1996, Reber 1993), which could reectimplicit learning stored in functionally par-

allel neural networks (Dijksterhuis & Smith2005; Smith & DeCoster 1999, 2000). How-ever, although the notion that System 2 isin some sense rule-based is compatible withthe proposals of most dual-process theorists,the characterization of System 1 as associativeis not. The problem, as already identied, isthat there are multiple systems of implicit pro-cesses and it is far from clear that the differ-ent theories can be mapped on to each at theSystem 1 end. In particular, theories that con-trast heuristic with analytic or systematic pro-cessing (Chen & Chaiken 1999, Evans 2006)seem to be talking about something differentfrom associative processing.

Other recurring themes in the writingof dual-process theorists are that System 1processes are concrete, contextualized, ordomain-specic, whereas System 2 processesare abstract, decontextualized, or domain-general. The notion that System 1 processesrapidly contextualize problems with priorknowledge and belief has been particularlyemphasized in dual-process accounts of hu-man reasoning (Evans 2006, Klaczynski &Lavallee 2005, Stanovich 1999). Such authorsassume that belief-based reasoning is the de-fault to which conscious effortful analytic rea-soning in System 2 may be applied to over-come. However, it may be unwise to deneSystem 2 as being abstract and decontextual-ized if we also want to retain its description asslow, sequential, explicit, and rule-based be-cause none of these characteristics may be lim-ited to abstract forms of reasoning (Sloman2002, Verschueren et al. 2005). It would prob-ably be more accurate to say that although ab-stract reasoning requires the use of System 2,concrete contexts do not preclude itsapplication.

The consideration that System 2 think-ing is not necessarily abstract and decontex-tualized is also one reason why it should notbe equated with a mental logic. The ideathat higher forms of thinking require a logicin the mind was popularized by Piaget (seeInhelder & Piaget 1958) and is particularly as-sociated with the idea that people have natural


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logics composed of inference rules in theirminds (Braine & OBrien 1998a, Rips 1994).However, the popular mental models theoryof deductive reasoning ( Johnson-Laird 1983,Johnson-Laird & Byrne 1991) can also be re-garded as a form of mental logic, accountingfor deductive competence by semantic ratherthan syntactic principles (Evans & Over 1996,Oaksford & Chater 1995). Whichever ac-count of deduction is preferred, it is clear thatthe System 2 concept is much broader thanthat of logical reasoning, including such ideasas an inhibitory role (suppressing pragmaticinuences of System 1) and the ability toengage in hypothetical thought via supposi-tion and mental simulations. This is prob-ably why most dual-process theorists preferbroader terms such as analytic or system-atic to describe the second system.

Individual Differences

Some dual-process theorists (Reber 1993,Stanovich 1999) have claimed a link betweenSystem 2 processing and general intelligence,with the corollary that System 1 processes areindependent of general intelligence. This pro-posal has led to an increasing use of individ-ual differences methodology in dual-processresearch, as revealed in the sections below. Inaddition to linking the effectiveness of ana-lytic reasoning and decision making with gen-eral intelligence measures, researchers havealso investigated two close correlates: work-ing memory and age of development. It is nowwell established that individual differences inworking memory capacity and general intel-ligence measures are very highly correlated(Colom et al. 2004). Working memory capac-ity is known to predict performance levels in avery wide range of cognitive tasks and has beendirectly linked with dual-process accounts ofcognitive functions, albeit primarily at a lowerlevel than those that form the focus of thecurrent review (Barrett et al. 2004). Develop-mental studies are also relevant, as the analyticthinking skills that contribute to performanceon general intelligence tests develop with age.

However, it has recently been claimed thatit is ability and not age that is the impor-tant predictor of analytic reasoning, whichmay explain some inconsistencies in devel-opmental research (Kokis et al. 2002). A fur-ther complication is that, as some researchersclaim, System 1 may develop in parallel withSystem 2 (Klaczynski 2000, 2001).

Evolutionary psychologists such as Tooby& Cosmides (1992) have emphasized thattheir main interest lies in explaining intelli-gence that is universal and optimized acrossthe human species. However, they have beenstrongly criticized (Stanovich 1999, Stanovich& West 2003) for downplaying the impor-tance of heritable characteristics, particularlythat of general intelligence, in their discus-sion of higher-order cognitive processes suchas reasoning and decision making. In general,one of the stronger bases for dual-systems the-ory is the evidence that controlled cogni-tive processing correlates with individual dif-ferences in general intelligence and workingmemory capacity, whereas automatic pro-cessing does not. It seems at least to indicatethat behavior may be controlled both with andwithout the use of executive working memoryresources.

An important distinction in the individualdifferences approach is that between measuresof cognitive capacity and dispositional think-ing styles. The difference is between whatpeople are able to do and what they are in-clined to do. Stanovich (1999), for example,shows that residual variance in solution ratesof reasoning and judgment task, when theeffects of cognitive ability have been takenout, can be accounted for in terms of disposi-tions for critical thinking. Evidence for cross-cultural differences in thinking styles (Nisbettet al. 2001) are also dispositional as they canalter when people move to another culture.In the social cognition literature, there hasbeen much attention to individual differencesin thinking style as measured by such scalesas need for cognition (Cacioppo & Petty1982) or the rational-experiential inventory(Epstein et al. 1996). It is important to note

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that the observation of two thinking styles initself does not constitute evidence for dualprocesses arising from two distinct cognitivesystems.

APPLICATIONS OFDUAL-PROCESS THEORIESIN DIFFERENT DOMAINS

As the preceding discussion illustrates, System2 appears to be a more coherent and consis-tent concept in the generic dual-system theorythan does System 1 because multiple systemsof implicit cognitive processes exist (Wilson2002, Stanovich 2004). It is also likely thatdifferent dual-process theorists have differentimplicit systems in mind and that these sys-tems do not have a single evolutionary his-tory. I return to the issue of whether genericdual-system theory is sustainable at the end ofthe review. At this point, I look more closelyat how dual-process theories are applied tothe explanation of particular phenomena inhigher cognition.

Dual-Process Theories of Reasoning

The psychology of deductive reasoning waslargely established by the work of PeterWason in the 1960s and 1970s (see Wason& Johnson-Laird 1972). Wason invented sev-eral famous tasksincluding the four-card se-lection taskthat are still used in current re-search. The eld has expanded rapidly overthe past 40 years and changed its characteras authors have become progressively less at-tached to the normative standard of formallogic and more interested in the inuence ofcontextual factors invoking prior belief andknowledge (Evans 2002). Dual-process ideasdeveloped quite early on, with the rst useof the term appearing in the title of a paperpublished by Wason & Evans (1975). This pa-per focused on the nding that card choiceson the selection task were strongly inuencedby a seemingly primitive matching bias (se-lecting cards explicitly mentioned in the con-ditional statement). Participants showed no

awareness of this in their verbal reports, in-stead justifying their choices with regard tothe experimental instructions to prove the ruletrue or false. Wason & Evans concluded thatparticipants were rationalizing with their con-scious reasoning causes of behavior that werein fact unconscious. Their arguments werevery much in accord with the famous critiqueof introspective reports presented by Nisbett& Wilson (1977).

The heuristic-analytic theory of reason-ing (Evans 1989) shifted the emphasis of theheuristic processes responsible for biases toa pragmatic and preconscious level that pre-ceded any attempt at analytic processing. Thistheory is narrower in scope than typical dual-process theories and is strongly focused on theexplanation of biases in reasoning and judg-ment tasks. The idea was that heuristic pro-cesses selectively focused attention on taskfeatures that appeared relevant, introducingrelevant prior knowledge in the process. Sinceanalytic processing could only be applied tothese selective representations, biases wouldbe observed when either (a) logically relevantinformation was excluded or (b) logically irrel-evant information was included by heuristicprocessing.

The sequential nature of the heuristic-analytic theory contrasts with parallel andinteractive forms of dual-process theory, al-though many of the proposed features ofthe two processes correspond to those of thegeneric System 1 and 2: Heuristic processesare fast, automatic, and belief based, whereasanalytic reasoning is slow, sequential, and canmake an effort at deduction. In a recent re-formulation of the theory, Evans (2006) hassought to reconcile it with conict models bythe proposal that heuristic responses can con-trol behavior directly unless analytic reason-ing intervenes. In other words, heuristics pro-vide default responses that may or may not beinhibited and altered by analytic reasoning.Analytic system intervention may be cued bystrong deductive reasoning instructions andmay be more likely to occur when individu-als have high cognitive ability or a disposition


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to think reectively or critically (Stanovich1999).

For much of its recent history, the psychol-ogy of deductive reasoning has been dom-inated by a debate between proponents ofmental logic theorists, who attribute deduc-tive competence to sets of abstract inferencerules (Braine & OBrien 1998a, Rips 1994),and mental model theorists ( Johnson-Laird1983, Johnson-Laird & Byrne 1991), who ex-plain it in terms of a fundamental semanticprinciple: An argument is valid if there areno counterexamples to it. Although the writ-ings of mental logic and mental model theo-rists include little explicit discussion of dual-process theory, the distinction is implicitlypresent in the theories. Mental logic theo-rists (for example, Braine & OBrien 1998b)make great play of the distinction between di-rect rules of inference, which are applied ef-fortlessly and accurately, and indirect rules,which require conscious effort and are muchmore error prone. They also explain errors inreasoning by reference to pragmatic implica-tures and other contextual effects. Similarly,the mental model theory describes the forma-tion of initial mental models as a relativelyautomatic and effortless process. The processof searching for counterexamples ( Johnson-Laird & Bara 1984) or the eshing-out of ini-tially implicit mental models ( Johnson-Laird& Byrne 2002) is, however, effortful and er-ror prone and constrained by working mem-ory capacity. Both theories include proposalsthat could account for pragmatic inuenceson reasoning and for the relation of reasoningaccuracy to individual differences in cognitivecapacity.

The paradigm case for dual processesin reasoning is belief bias. In the standardparadigm, people are given syllogisms andasked to evaluate their logical validity. Syl-logisms vary in both their validity (whetherthe conclusion follows from premises) andtheir believability (whether the conclusionconforms or conicts with prior belief). Thus,some syllogisms are belief-logic compatible,but some provide conictvalid arguments

with unbelievable conclusions and invalid ar-guments with believable conclusions. The ba-sic phenomena of this paradigm were estab-lished by Evans et al. (1983), whose ndingshave been replicated many times since (seeKlauer et al. 2000 for a recent extensive study).There is (a) a main effect of logic, in that validconclusions are more often accepted than in-valid conclusions, (b) a main effect of belief(belief bias) in that believable conclusions aremuch more often accepted, and (c) a belief bylogic interaction, in that belief bias is muchmore marked on invalid syllogisms.

In the original study, Evans et al. (1983)argued, on the basis of protocol analyses andthe examination of individual response pat-terns, that there was a within-person conictbetween a logical and belief-based reasoningprocess. They offered two explanations of thebelief by logic interaction, which were lateraugmented by one derived from mental modeltheory (Oakhill et al. 1989). More recently,accounts of the phenomena have been devel-oped that are strongly compatible with dual-process theory (e.g., Klauer et al. 2000). Theseaccounts are supported by evidence of a shiftfrom logical to belief-based reasoning undersevere time pressure (Evans & Curtis-Holmes2005) and under concurrent working memoryload (De Neys 2006).

There has been considerable interest inhow people reason on belief-logic conictproblems; that is, the valid-unbelievable andinvalid-believable syllogisms. Logical perfor-mance on such cases is known to decline withage (Gilinsky & Judd 1994) and to be re-lated to individual differences in cognitiveability (Kokis et al. 2002, Newstead et al.2004, Stanovich & West 1997). Stanovich(1999) argued on this basis that participantsof higher cognitive ability are more able to in-hibit belief-based reasoning. However, avail-able data are more consistent with the viewthat although high-ability people reason morelogically with belief-laden (as well as abstract)syllogisms, the belief bias effect is no lessmarked for them than for lower-ability peo-ple (Klaczynski 2000, Newstead et al. 2004,

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Torrens et al. 1999). Thus, it may be thathigher-ability people do not engage in moreSystem 2 reasoning but rather are simply moresuccessful when they do so.

A large literature exists on how people rea-son with the Wason four-card selection task(see Evans & Over 2004, Chapter 5, for areview). The task requires people to decidewhich of four cards to turn over to decide thetruth of a conditional statement. For exam-ple, if the statement says, If a card has an Aon one side then it has a 3 on the other, andthe visible sides display A, D, 3, and 7, then thelogically correct choice is the A and the 7. Thisis because only a card that has an A and not a 3could disprove the statement. Few people givethis answer; most are more likely to choose Aand 3, or just A. Although this abstract, indica-tive form of the task is very difcult, concreteand deontic forms (concerning rules and reg-ulations) are much easier. For example, giventhe statement, If a person is drinking beerin a bar, that person must be over 19 years ofage, most participants readily see that theymust check beer (rather than soda) drinkersand those who are under 19 years of age.

The heuristic-analytic theory explains thedifculty of the abstract task on the groundsof heuristics that focus attention on selectedcards, a claim supported by the time that peo-ple spend observing individual cards (Evans1996, but see Ball et al. 2003, Roberts 1998)and the accentuation of matching bias by arequirement to respond rapidly (Roberts &Newton 2001). Stanovich & West (1998) haveproduced strong evidence that analytic rea-soning is involved in solving the abstract se-lection task, since solvers have much higherSAT scores than do nonsolvers. The relationto cognitive ability is, however, much weakerfor concrete and deontic forms of the task,suggesting that it can be solved by pragmaticbelief-based reasoning in System 1 (see alsoNewstead et al. 2004).

Recently, various authors have applieddual-process theory to the inferences peopledraw from conditional statements. Consider-able evidence shows that reasoning with re-

alistic conditionals is strongly inuenced bypragmatic factors, including the availability ofcounterexamples to inferences from seman-tic memory (Cummins et al. 1991; Thompson1994, 2000) and the strength of association ofcounterexamples as measured across both ma-terials and individual participants (De Neyset al. 2005, Markovits et al. 1998, Markovits &Quinn 2002). Participants are also more likelyto draw inferences from conditionals thatthey believe rather than disbelieve (George1997; Liu et al. 1996; Newstead et al. 1997;Stevenson & Over 1995, 2001). However, ev-idence suggests that analytic System 2 reason-ing processes, as well as heuristic System 1processes, may be inuenced by prior beliefabout the problem context (Verschueren et al.2005).

In conclusion, dual-process theory hasbeen widely applied to the study of syllogis-tic reasoning, the Wason selection task, andconditional inference. (For a critical reviewof the theory in the psychology of reasoning,see Osman 2005.) Experimental evidence hasconsistently shown that responses are partiallyconsistent with logic but are also inuenced bysystematic biases such as matching bias andbelief bias. Cognitive models have generallydepicted these as competing inuences in awithin-participant conict. Several differentforms of evidence support dual-processing ac-counts, including (a) the observation of morelogical and less belief-based reasoning un-der strong deductive reasoning instructions,(b) the association (in general) of better log-ical accuracy with higher-ability participantswhen problems cannot also be solved by apragmatic route, and (c) the nding that work-ing memory load or instructions to respondrapidly increase levels of typical biases as wellas reduce logical accuracy.

Dual-Process Theories of Judgmentand Decision Making

Three main research paradigms have domi-nated the psychology of judgment and deci-sion making: (a) the heuristics and biases


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research program that is focused particularlyon judgments of probability (Gilovich et al.2002, Kahneman et al. 1982), (b) the studyof decision making under risk (Wu et al.2005), and (c) social judgment theory and thelens model (Doherty 1996). The last of theseparadigms, which may be less familiar to read-ers, is focused on judgments made where mul-tiple cues are available in the environment. Anexample might be medical diagnosis in lightof a number of demographic factors, patienthistory, symptoms, diagnostic tests, and so on.It is possible to capture the implicit policy ofjudges by using multiple regression analysis toshow which of the various cues predict theiroverall judgment.

Until recently, dual-process theory playeda much smaller part in these elds than inthe study of reasoning and social cognition.An exception is the cognitive continuum the-ory of Hammond (e.g., 1996), working in so-cial judgment theory, which contrasts intuitiveand analytic thinking. However, as the namesuggests, this theory proposes two ends of acontinuum rather than discrete processes orsystems and hence is not the main type of the-ory with which this review is concerned.

Kahneman & Frederick (2002, 2005) re-cently developed a dual-process theory ofprobability judgment that they link to thegeneric dual-system theory. Heuristic judg-ments, which lead to biases, are associ-ated with System 1, and analytic reasoning,which may intervene with these judgmentsand improve them, are linked to System 2.This helps explain general ndings that al-though biases in probability judgment canbe linked to heuristics such as representa-tiveness (Kahneman & Tversky 1972, Teigen2004) and availability (Reber 2004, Tversky& Kahneman 1973), these biases are not uni-versally observed but do appear to competewith a tendency to give normatively correctanswers. Such conicts stand in parallel withthe ndings in the deductive reasoning litera-ture, reviewed above, in which matching andbeliefs biases compete with logically correctanswers. In general, normatively correct solu-

tions to problems from both literatures tendto be found more often by those of higher cog-nitive capacity (Stanovich 1999), who may beassumed to make more effective interventionswith analytic or System 2 reasoning processes.

The processing assumptions of Kahneman& Fredericks theory are similar to those of theheuristic-analytic theory of reasoning (Evans2006). It is assumed that fast System 1 (heuris-tic) processes cue default intuitive judgmentsthat must nevertheless be endorsed by the(analytic) System 2, which often does so ca-sually. This may involve attribute substitu-tion, in which people actually answer a differ-ent (and easier) question than the one asked.However, high-effort deliberative reasoningmay be applied, which can inhibit the bi-ased response and replace it with one basedon reective reasoning. I call this kind ofdual-process theory default-interventionistas compared with theories (e.g., that ofSloman, 1996) that are parallel-competitivein nature. Kahneman & Frederick suggest thateven heuristics that are apparently consciousin application, such as the recognition heuris-tic (Gigerenzer et al. 1999), have an automaticcomponent. In this case, feelings of familiar-ity are automatically recruited and then con-sciously interpreted as a basis for making ajudgment, such as the relative size of foreigncities.

A major recent debate in the probabil-ity judgment literature has concerned theclaim that probability problems are much eas-ier if framed in terms of frequencies ratherthan in probabilities (Barbey & Sloman 2007,Cosmides & Tooby 1996, Gigerenzer &Hoffrage 1995), thus reducing biases suchas base rate neglect in Bayesian reasoning(Kahneman & Tversky 1973, Koehler 1996).It was argued from an evolutionary perspec-tive that we would have evolved a cognitivemodule for processing frequency informationin the environment (Cosmides & Tooby 1996)that could not be applied to one-case proba-bilities. However, from a dual-process view-point, it appears that such a module wouldaffect learning behavior in System 1 rather

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than in explicit System 2 reasoning about thequantitative word problems that are actuallypresented (Evans et al. 2000). Consistent withthis, there is now much evidence that whatfacilitates Bayesian reasoning is a problemstructure that cues explicit mental models ofnested-set relationships (Evans et al. 2000,Girotto & Gonzalez 2001, Sloman et al.2003) as originally proposed by Tversky &Kahnemant (1983). However, Hoffrage et al.(2002) have responded by arguing that suchnested sets are intrinsic to natural sampling, aprocess that leads to the observation of naturalfrequencies that encode base rate informationimplicitly. Hence, they claim that Bayesianposterior probabilities can be derived fromdirect comparison of such frequencies withno difcult calculation involved. However,Barbey & Sloman (2007) have recently arguedthat this kind of evolutionary account, whenexamined in detail, is a good deal less parsi-monious than an explanation in terms of dualprocessing.

Discussion of work on reasoning and judg-ment to this point may have suggested thatSystem 2 processing is in some sense supe-rior to that of System 1, in that the formeris often associated with normatively correctresponding and the latter with cognitive bi-ases. However, those authors who have lookedat expert judgment and decision-making pro-vide a somewhat different perspective. For ex-ample, in Kleins (1999) naturalistic studies ofdecision making in groups such as re of-cers and paramedics, the author argues thatvery little rational decision-making goes on,in the sense of deliberation between alterna-tives. What typically happens is that an expertrecognizes a situation as of a kind encounteredpreviously and rapidly retrieves a schema thatprovides a solution, a process Klein termsrecognition-primed decision-making. Theapplication will involve some explicit reason-ing (sometimes mental simulations to checkfeasibility of solutions), but the key to intelli-gent action is the automatic retrieval process.

The value of System 1 processing has alsobeen emphasized by other dual-process the-

orists in applications to decision-making. Forexample, Reyna (2004) argues that experts ac-quire gist knowledge that allows them to makeintuitive responses that are automatic, rapid,and effective, whereas novices need to rely onexplicit analytic reasoning. However, Reynanotes also that the former kind of process canlead to bias and error when novel problemsare presented, as is typically the case in lab-oratory studies of probability judgment. Per-haps more controversial is the recent claim ofDijksterhuis et al. (2006) that there are pro-cesses of conscious and unconscious reason-ing, both of a deliberative nature, and that theunconscious reasoning leads to superior de-cision making. This is a very different kindof claim from those of the theories we havebeen considering that envisage fast heuris-tic processes delivering contextualized con-tent for evaluation by a conscious, analyticprocess.

The evidence for superiority of uncon-scious decision-making is that people maymake better decisions as measured by norma-tive analysis or by correspondence with expertjudgments (Wilson & Schooler 1991) whenconscious deliberation is prevented by short-age of time or by competing tasks. This ap-plies to fairly complex, multiattributed de-cision problems. However, it is unclear whyany unconscious process of deliberative rea-soning need be postulated. The evidence isconsistent with the claim that where partic-ipants have a history of relevant experien-tial learning, fast recognition processes mayprovide accurate intuitive responses. Whatis interesting, however, is that conscious de-liberative reasoning may then interfere withgood decision-making. It may be that this re-ects its sequential and low-capacity nature.An analogous nding in the learning literatureis that complex rule learning may sometimesbe inhibited by an instructional set for explicitlearning (Reber 1993).

Dual processes would seem to be impli-cated when we contrast intuitive judgmentwith reective decision-making. Many every-day decisions seem to involve rapid intuitive


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judgments in which courses of action springto mind with little or no effort of consciousthinking. Much expert decision-makingseems to have this character (Klein 1999). Onthe other hand, we can and do make somedecisions in a manner much more akin to thatprescribed by decision theory, exploring al-ternative actions and their consequences withextended mental simulations (Kahneman &Tversky 1982). Intuitive judgments seem tohave the System 1 characteristics, whereasreective decision-making seems much morelike a System 2 process (see also Kahneman& Frederick 2002). Other examples familiarto us all concern phobias and compulsivebehaviors like gambling, overeating, orsmoking, where we may become aware ofa System 1 and 2 conict. We may judgeour own behavior or that of others to beirrational because we compulsively behavein ways that are at odds with our explicitlystated (System 2) goals. Some authors havedescribed these kinds of phenomena as im-plying two minds in one brain (Evans 2003)or a brain at war with itself (Stanovich 2004).

Dual-Process Theories of SocialCognition

Dual-process theories of social cognitionemerged in the 1980s (Chaiken 1980, Petty &Cacioppo 1981) and developed in popularityto form the dominant paradigm for the past20 years or more. Contemporary work par-ticularly concerns the automatic and uncon-scious processing of social information in suchdomains as person perception, stereotyping,and attitude change (Bargh 2006, Chaiken &Trope 1999, Forgas et al. 2003, Hassin et al.2005, Smith & DeCoster 2000, Wilson 2002)and its apparent dissociation from explicit be-liefs and conscious processing. The proposalof new accounts or at least new labels for dualprocesses in social cognition has reached nearepidemic proportions, causing some reactionin terms of a unimodel that instead empha-sizes multiple parameters known to inuencesocial judgments (Kruglanski et al. 2003).

Another interesting recent development isthe quad model of Conrey et al. (2005), whichproposes four kinds of process that should bedistinguished in order to interpret researchon automatic and controlled cognition.

Dual-processing accounts of social cogni-tion have their roots in cognitive psychology,especially the study of automaticity and im-plicit memory, but have made curiously littleconnection with the dual-process theoriesof reasoning and decision-making discussedabove. Although there is currently very littlecross-referencing between these literatures,dual-process accounts in social psychologynevertheless share many common featureswith those in the cognitive psychology ofreasoning and judgment. However, there aredifferences of emphasis. In general, the socialcognition literature is less concerned withissues about cognitive architecture and evo-lution but more focused on issues concerningconsciousness, free will, and the implica-tions for moral and legal responsibilities ofindividuals.

A long-established dual-process theory isthe heuristic-systematic model of Chaiken.According to Chen & Chaiken (1999, p.74; Petty & Cacioppo 1981) Systematic pro-cessing entails a relatively analytic and com-prehensive treatment of judgment relevantinformation . . . . Given its nature, system-atic processing requires both cognitive abil-ity and capacity whereas Heuristic process-ing entails the activation and application ofjudgmental rules and heuristics that are pre-sumed to be learned and stored in mem-ory . . . . Relative to systematic processing,heuristic processing make minimal cognitivedemands on the perceiver. Systematic pro-cessing seems a similar concept here to thatof analytic or System 2 reasoning in theo-ries described above. However, heuristic pro-cessing in this theory sounds more like therecognition-primed decision making of Klein(1999) than the contextualization process pos-tulated by reasoning theorists (Evans 2006,Stanovich 1999). In fact, heuristic process-ing so dened could be taken to be a form

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of System 2 or rule-based reasoning, albeitone less effortful than that which is calledsystematic processing (see Strack & Deutsch2004).

The cognitive experimental self theory, orCEST (Epstein 1994, Epstein & Pacini 1999),proposes two cognitive systemsexperientialand rationalthat share many common fea-tures with the generic two-system theory ofreasoning. Like System 1, the experiential sys-tem is described as having a long evolutionaryhistory with clear links to animal cognition,whereas the rational system, like System 2, isrecent and distinctively human. Many otherfeatures of the generic theory shown inTable 1 are included: fast-slow, unconscious-conscious, associative-rule based, and so on,whereas emotion is explicitly linked to the ex-periential system. However, the theory hasa parallel-competitive, rather than default-interventionist, structure, and it includes theproposal that each system has access to dis-tinct forms of knowledge. Epsteins approachis distinctive in the linkage of the two systemsto two competitive processing styles. In con-trast with reasoning theorists like Stanovich,who keep a clear separation between cogni-tive systems and dispositional thinking styles,Epstein has developed a psychometric tool,the rational experiential inventory, or REI(Epstein et al. 1996), for measurement of thetwo styles based on self-report. Epstein &Pacini (1999) review a number of experimen-tal studies that appear to support the existenceof these two processing styles. However, nei-ther people classied as rational thinkers northose scoring highly on the related need-for-cognition scale appear to have any advantageon abstract tests of logical reasoning (Borset al. 2006, Newstead et al. 2004).

Social psychologists have been particularlyinterested in links between unconscious pro-cessing and implicit forms of knowledge rep-resentation. For example, it has been pro-posed that people may have both implicitand explicit attitudes (Wilson et al. 2000),something that may help to explain the tra-ditional dissociation between verbal and be-

havioral measure of attitude changes. It maybe possible to change our explicit attitudewhile an implicit attitude continues to con-trol our social behavior. Similarly, it has beenargued that we may have both implicit andexplicit stereotypes that are dissociated fromone another (Bargh 1999, Bargh & Williams2006, Devine 1989). Many studies have usedthe methodology of semantic priming, bor-rowed from the literature on implicit memory(Lucas 2000). For example, Macrae et al.(1997) showed that when people are asked toview photographs of males or females and re-quired to process them in a semantic manner,their subsequent threshold for word recog-nition is primed for stereotype-consistentwords. Although some authors have proposedan optimistic view that conscious processingcan inhibit implicit stereotypes, the evidencesuggests the contrary. Even people who havenonstereotypical explicitly stated beliefs andwho are aware of the problem of stereotypicalbehavior can be shown experimentally to havemuch of their social behavior unconsciouslycontrolled (Bargh 1999).

A recent attempt to link dual-processaccounts in social cognition with those incognitive psychology was made by Smith &DeCoster (2000), who build on the distinctionof two kinds of memory, one based on slowacquisition through associative learning andone linked to explicit memory (McClellandet al. 1995). These led them to a parallelsystem account in which associative andrule-based processing (cf. Sloman 1996) arelinked to the two forms of knowledge. Thisproposal is essentially similar to dual-processaccounts that have been developed to distin-guish implicit and explicit forms of learning(Berry & Dienes 1993, French & Cleeremans2002, Reber 1993, Sun et al. 2005). Smith &DeCoster argue that the major dual-processtheories in social psychology can be accom-modated within this general framework.However, as noted earlier, this is doubtful inthe case of the heuristic-systematic theory,which looks more like two forms of rule-basedprocessing (Strack & Deutsch 2004).


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An important development for dual-process and dual-system theory generally isthe emergence of social cognitive neuro-science. Particularly interesting is the iden-tication of reexive (System 1) and re-ective (System 2) cognitive processing,with two neurological systems described asthe X-system and C-system, respectively(Lieberman 2003, Lieberman et al. 2004).The X-system is composed of the amygdala,basal ganglia, and lateral temporal cortex,brain areas known to be involved in con-ditioning and associative learning and nowbeing linked by Lieberman and colleagueswith social cognitive processes tradition-ally described as automatic or implicit. TheC-system involves the anterior cingulatecortex, prefrontal cortex, and the medial-temporal lobe (including hippocampus), brainareas known to be involved with (among otherthings) explicit learning and inhibitory, exec-utive control. This account is supported byrecent ndings concerning the neurologicalsystems that underlie response to immedi-ate and deferred rewards in decision-making(McClure et al. 2004), which correspond to X-and C-system regions of the brain. Althoughin its early stages, this research program pro-vides perhaps the strongest basis in the lit-erature for maintaining some form of dual-system distinction.

A nal issue to note in this section is thatof self-knowledge. Although the notion thatSystem 2 reasoning may engage in ratio-nalization or confabulation is mentioned inthe cognitive literature (Evans & Over 1996,Stanovich 2004), this idea has been morethoroughly investigated in the social psychol-ogy literature, especially by Wilson and hiscolleagues (Nisbett & Wilson 1977, Wilson2002, Wilson & Dunn 2004). The basic ideahere is that although much of our behavioris unconsciously controlled, we (consciousbeings) are not aware of this fact and may livewith an illusion that we are much more in con-trol of our behavior than we actually are. Onthis view, we observe and theorize about ourown social behavior in much the same way

as we attempt to perceive and understand thebehavior of others.

CONCLUSIONS

Although dual-process theories have beenaround in cognitive and social psychology for30 years and more, it is only within the past10 years or so that the terms System 1 andSystem 2 have come into common use. Sopopular are these terms now that it may besomewhat difcult to discourage their use andthe implication of two underlying generic sys-tems that they convey. However, close in-spection of the evidence suggests that genericdual-system theory is currently oversimpliedand misleading. In particular, (a) it is not pos-sible coherently to link together all the at-tributes associated with Systems 1 and 2, re-spectively, in Table 2, certainly when movingbetween clusters, and (b) there are at least twoquite distinct forms of dual-process theory tobe found in these various literatures that can-not readily be mapped on to each other.

We might be better off talking about type 1and type 2 processes since all theories seem tocontrast fast, automatic, or unconscious pro-cesses with those that are slow, effortful, andconscious (Samuels 2006). Such terminologydoes not commit use to a two-system view.However, it would then be helpful to havesome clear basis for this distinction. If we can-not associate all the System 1 (or type 1) fea-tures shown in Table 2 together, for exam-ple, then which are the key ones that shoulddistinguish them from System 2 (or type 2)processes? My suggestion is that type 2 pro-cesses are those that require access to a single,capacity-limited central working memory re-source, while type 1 processes do not requiresuch access. This implies that the core fea-tures of type 2 processes are that they are slow,sequential, and capacity limited. The last fea-ture implies also that their functioning willcorrelate with individual differences in cogni-tive capacity and be disrupted by concurrentworking memory load. Depending upon whatelse is assumed about working memory, there

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may be a rationale for describing such type 2processes as registering in consciousness andhaving properties associated with executiveprocesses and intentional, higher-order con-trol. However, other proposed features ofSystem 2 in the generic theory do not imme-diately follow from this denition of type 2processes, for example, the proposal that suchprocesses are uniquely human or associatedwith decontextualized thought or rule-basedreasoning.

The problem with this distinction is thattype 1 processes then simply refer to any pro-cesses in the mind that can operate auto-matically without occupying working mem-ory space. As already indicated, there are anumber of different kinds of such implicit pro-cesses. We may have innate cognitive mod-ules with encapsulated processes for percep-tion, attention, language processing, and soon. We appear to have an associative learningsystem that implicitly acquires knowledge ofthe world in a form similar to weights in neuralnetworks; the knowledge cannot be called tomind as explicit knowledge, but it can directlyaffect our behavior. We have habitual and au-tomated behavior patterns that once requiredconscious type 2 effort but seem to have be-come type 1 with practice and experience. Wealso have powerful pragmatic processes thatrapidly identify and retrieve explicit knowl-edge for conscious processing. Type 2 pro-cessing requires supporting type 1 processesto supply a continuous stream of relevant con-tent into working memory.

If there are indeed multiple kinds of type 1processes, then it is to be expected that psy-chologists will have developed different kindsof dual-process theories, which seems to bethe case. Parallel-competitive forms of dual-process theory seem to be rooted in the ideaof two forms of learning, leading to two formsof knowledge (implicit and explicit) that canthen lead to competing attempts to controlbehavior. Theories of this type include thoseof Sloman (1996), Reber (1993), and Smith& DeCoster (2000). As mentioned above,

there is promising evidence that these theo-ries can be mapped onto neurologically dis-tinct X- and C-systems (Lieberman 2003).However, the category of theories that I calldefault-interventionist assume, in contrast,that rapid preconscious processes supply con-tent for conscious processing, cueing defaultbehaviors that the analytic reasoning may ap-prove or intervene upon with more effortfulreasoning. This approach is reected in dual-process theories of reasoning (Evans 2006,Stanovich 1999) as well as the theory of in-tuitive and reective judgment proposed byKahneman & Frederick (2002). If there areindeed two parallel cognitive and neurologicalsystems, it is possible either that (a) the latterclass of theories are mistaken in their archi-tectural assumptions or (b) they are dealingwith interactions between preconscious andconscious elements of the second system.

In short, my conclusion is that althoughdual-process theories enjoy good empiricalsupport in a number of elds of psychology,the supercially attractive notion that they areall related to the same underlying two systemsof cognition is probably mistaken, at least inthe way that Systems 1 and 2 are being de-ned in the current literatures. For example,it is almost certainly wrong to think of System1 as one system, all of which is old and sharedwith other animals. Equally, it is probably amistake to think of System 2 as the consciousmind, all of whose processes are slow and se-quential. If there is a second system, distinc-tively human, involving working memory andneurologically distinct structures, it does notfollow that all of its workings are consciousand controlled. It is perfectly possible that onesystem operates entirely with type 1 processesand that the other includes a mixture of type1 and type 2 processes, the latter being linkedto the use of working memory, which thissystem usesamong other resources. Such aproposal could resolve the conict betweenevidence for dual systems on the one handwith the proposals of different dual-processtheorists on the other.


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ACKNOWLEDGMENTS

The writing of this review was supported by a Research Fellowship award to the author bythe Economic and Social Research Council of the United Kingdom (RES 00027-0184). Theauthor is grateful to Keith Stanovich, Tim Wilson, Phil Core, Eliot Smith, Shira Elqayam, andYaniv Hanoch for their comments on an earlier draft of this review.

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