Barbey & Sloman (B&S) argue that evolutionary accounts of base-rate neglect are unparsimonious, especially with regard to the facilitatory effects of frequency formats. We agree. They also propose that the phenomena can be best accounted for with a dual-processing framework. Here we also agree, in general, but not with the specific dual-processing account offered by these authors. This is based upon Sloman's (Reference Sloman1996a) proposal of two parallel systems for reasoning, one associative and one rule-based. The impression that B&S give, that this theory is representative of dual-process models of reasoning and judgement in general, is not correct.
Parallel-form dual process theories occur commonly where authors propose two forms of knowledge. This applies in dual-process accounts of learning (Reber Reference Reber1993; Sun et al. Reference Sun, Slusarz and Terry2005) where implicit and explicit learning processes lead to something like associative neural nets and propositional rules, respectively. This form of theory is also common in social psychology (see Smith & DeCoster Reference Smith and DeCoster2000) where researchers are often concerned to distinguish implicit stereotypes and attitudes, inferable from behaviour, from their explicit counterparts that are verbally stated. Other dual-process theories of reasoning and judgement, however, have a sequential form in which the implicit processing is pragmatic rather than associative and serves to contextualise explicit analytic thinking. This is the type of account offered by Stanovich (Reference Stanovich1999) and by Evans (Reference Evans2006). In these theories, System 2 monitors default intuitions arising in System 1 and may intervene with more effortful and abstract reasoning (see also Kahneman & Frederick Reference Kahneman, Frederick, Gilovich, Griffin and Kahneman2002).
We actually think it quite plausible, as Cosmides and Tooby (Reference Cosmides and Tooby1996) argue, that both humans and other animals would have evolved mechanisms for processing frequency information about their environments. What we find implausible is that any such cognitive module would be applied to quantitative word problems of the kind presented in experiments on Bayesian inference. In particular, we note that advocates of the frequency-processing module do not ask their participants to engage in experiential learning of frequencies, but present them with collated frequencies instead. Under such circumstances, people can only apply general-purpose procedures for reasoning by construction and manipulation of mental models. As B&S demonstrate in their review, this will only be successful when people are cued to construct models which encode the nested set relationships.
We can find no evidence in the target article for the authors' assertion that base-rate neglect is due to associative processing. What association is involved? That between the hypothesis and the diagnostic information, presumably – which would be a similar account to the one given in terms of representativeness (Kahneman & Tversky Reference Kahneman and Frederick1973). However, although the majority tendency may be to base an answer on diagnostic information, a substantial minority give the base rate as the answer instead (Cosmides & Tooby Reference Cosmides and Tooby1996; Evans et al. Reference Evans, Handley, Perham, Over and Thompson2000). Also, when real-world beliefs rather than stated probabilities are used to convey Bayesian priors, they may have much stronger influence than the diagnostic data on the judgments made (Evans et al. Reference Evans, Handley, Over and Perham2002, Experiment 5). What is common to all these cases is a failure to integrate base rate and diagnostic information, which should be equally weighted. Only when set-inclusive mental models are cued, will analytic reasoning be successful.
In this article, B&S clearly imply that application of System 2 reasoning will lead to correct responding. For example, they state that, “An appropriate representation can induce people to substitute reasoning by rules with reasoning by association” (sect. 4, para. 2). There has been a tendency for authors in the reasoning and judgement area to equate heuristic responding with biases and analytic reasoning with normatively correct responding, but this is a mistake that has been corrected in more recent dual-process accounts of reasoning (Evans Reference Evans2007; Stanovich Reference Stanovich, Evans and Frankish2006). In these new accounts, biases can occur in System 2 as well as System 1. The problem is that no psychologically plausible definition of System 2 reasoning – say, as slow, effortful, and engaging executive working memory – can immediately imply that it will be successful in finding normatively correct solutions. Even if it is correct to think of such reasoning as “rule based,” people may access rules that are normatively unsound, or fail to apply them in an appropriate manner. At best we could say that System 2 reasoning is necessary for logical reasoning; we certainly could not say that it is sufficient.
The literatures on reasoning and decision making are marked by confusion between competence or computational level accounts of reasoning and accounts based on normative rationality (see Elqayam Reference Elqayam, Vosniadou, Kayser and Protopapas2007). Even if we compare people of high and low cognitive ability, it is not universally the case that higher ability participants give more normatively correct answers. Recent experiments, for example, have shown that the logically valid modus tollens in inference may be endorsed less often by participants with higher ability (Newstead et al. Reference Newstead, Handley, Harley, Wright and Farelly2004). More directly relevant in the current context is the report by Stanovich and West (Reference Stanovich1998b) that participants who incorporated base rates had lower intelligence scores than participants who neglected them. It is clearly impossible to reconcile the following propositions: (a) System 2 necessarily produces normatively correct responses; (b) attention to base rate is a normatively correct response, and (c) System 2 is more strongly employed by those of higher ability. Of these three statements, (b) holds by definition, and there is overwhelming empirical evidence for (c). The only possible conclusion is that (a) is false.
In our view, the literature on base-rate neglect is amenable to a dual-processing account, but not of the kind proposed by B&S. We suggest that university student participants, instructed to engage in reasoning, will in fact do so but with varying degrees of conformity to Bayesian inference. Without formal training they will have no access to the rules of Bayesian inference and can therefore only attempt to use general-purpose analytic reasoning procedures which involve constructing and manipulating mental models to represent the problem information. With standard presentations, it will appear to them that either base rate or else (more commonly) diagnostic information is relevant, but they will have no means of integrating the two. Only when problem design cues them to construct set-inclusive mental models will they succeed in computing the normatively correct solution.
