Much research in the last two decades has demonstrated that human responses deviate from the performance deemed normative according to various models of decision making and rational judgment (e.g., the basic axioms of utility theory). This gap between the normative and the descriptive can be interpreted as indicating systematic irrationalities in human cognition. However, four alternative interpretations preserve the assumption that human behavior and cognition is largely rational. These posit that the gap is due to (1) performance errors, (2) computational limitations, (3) the wrong norm being applied by the experimenter, and (4) a different construal of the task by the subject. In the debates about the viability of these alternative explanations, attention has been focused too narrowly on the modal response. In a series of experiments involving most of the classic tasks in the heuristics and biases literature, we have examined the implications of individual differences in performance for each of the four explanations of the normative/descriptive gap. Performance errors are a minor factor in the gap; computational limitations underlie non-normative responding on several tasks, particularly those that involve some type of cognitive decontextualization. Unexpected patterns of covariance can suggest when the wrong norm is being applied to a task or when an alternative construal of the task should be considered appropriate.

Three fallacies in the rationality debate obscure the possibility for reconciling the opposed camps. I focus on how these fallacies arise in the view that subjects interpret their task differently from the experimenters (owing to the influence of conversational expectations). The themes are: first, critical assessment must start from subjects' understanding; second, a modal fallacy; and third, fallacies of distribution.

Stanovich & West argue that their observed positive correlations between performance of reasoning tasks and intelligence strengthen the standing of normative rules for determining rationality. I question this argument. Violations of normative rules by cognitively humble creatures in their natural environments are more of a problem for normative rules than for the creatures.

Stanovich & West's dual-system represents a major development in an understanding of reasoning and rationality. Their notion of System 1 functioning as a computational escape hatch during the processing of complex tasks may deserve a more central role in explanations of reasoning performance. We describe examples of apparent escape-hatch processing from the reasoning and judgement literature.

Stanovich & West (S&W) have two goals, one concerned with the evaluation of normative models, the other with development of prescriptive models. Individual differences have no bearing on normative models, which are justified by analysis, not consensus. Individual differences do, however, suggest where it is possible to try to improve human judgments and decisions through education rather than computers.

Stanovich & West interpret errors in syllogistic reasoning in terms of computational limitations. I argue that the variety of strategies used by reasoners in solving syllogisms requires us to consider also performance errors. Although reasoners' performance from one trial to another is quite consistent, it can be different, in line with the definition of performance errors. My argument has methodological implications for reasoning theories.

A broad evolutionary perspective is essential to fully reverse figure and ground in the rationality debate. Humans' evolved psychological architecture was designed to produce inferences that were adaptive, not normatively logical. This perspective points to several predictable sources of errors in modern laboratory reasoning tasks, including inherent, systematic biases in information-processing systems explained by Error Management Theory.

A problem with Stanovich & West's inference that there a nonintellectual processing system independent from an intellectual one from data in which they partialled out global intelligence is that they may have controlled for the wrong kind of intellectual intelligence. Research on cognitive-experiential self-theory over the past two decades provides much stronger support for two independent processing systems.

A prospective integration of evolutionary and other approaches to understanding rationality, as well as incorporation of individual difference concerns into the research agenda, are major contributions of Stanovich & West's analysis. This commentary focuses on issues of concern in detailing a dual-system or dual-process model of the sort they propose and using it as a basis for improving judgment.

I discuss several problems with Stanovich & West's research and suggest an alternative way to frame the rationality debate. The debate is about whether analytic (System 2) thinking is superior to intuitive (System 1) thinking or whether the two modes are complementary. I suggest that the System 1/System 2 distinction is equivalent to the Chinese concepts of yin and yang and that the two modes are complementary.

The target article's finding of stable and general individual differences in solving of problems in heuristics-and-biases experiments is fundamentally subversive to the Meliorist research program's attention-getting claim that human thought is “systematically irrational.” Since some people get these problems right, studies of heuristics and biases may reduce to repeated demonstrations that difficult questions are hard to solve.

I discuss an aspect of individual differences which has not been considered adequately in the target article, despite its potential role in the rationality debate. Besides having different intellectual abilities, different individuals may produce different erroneous responses to the same problem. In deductive reasoning, different response patterns contradict deterministic views of deductive inferences. In decision-making, variations in nonoptimal choice may explain successful collective actions.

Stanovich & West continue a history of norm-setting that began with deference to reasonable people's opinions, followed by adherence to probability theorems. They return to deference to reasonable people, with aptitude test performance substituting for reasonableness. This allows them to select independently among competing theories, but defines reasoning circularly in terms of aptitude, while aptitude is measured using reasoning.

Stanovich & West (S&W) appear to overlook the adaptivity of variation. Behavioral variability, both between and within individuals, is an absolute necessity for phylogenetic and ontological adaptation. As with all heritable characteristics, inter-individual behavioral variation is the foundation for natural selection. Similarly, intra-individual variation allows a broad exploration of potential solutions. Variation increases the likelihood that more optimal behaviors are available for selection. Four examples of the adaptivity of variation are discussed: (a) Genetic variation as it pertains to behavior and natural selection; (b) behavioral and cognitive aspects of mate selection which may facilitate genetic diversity; (c) variation as a strategy for optimizing learning through greater exploration; and (d) behavioral variation coupled with communication as a means to propagate individually discovered behavioral success.

Can the understanding/acceptance principle help us to decide between alternative normative theories? There is little evidence that this principle can successfully be applied; there are no strong normative statements in Stanovich & West's target article. There is also no evidence for success of rational norms when applied to real life decisions.

The notion of “cognitive ability” leads to paradoxical conclusions when invoked to explain Inhelder and Piaget's research on class inclusion reasoning and research on the inclusion rule in the heuristics-and-biases program. The vague distinction between associative and rule-based reasoning overlooks the human capacity for semantic and pragmatic inferences, and consequently, makes intelligent inferences look like reasoning errors.

Stanovich & West claim that the positive correlation between reasoning tasks negates the view that errors in reasoning are due to failures in information processing. This is not correct. They conjecture that errors in reasoning are associated with conflicts between intentional and associative reasoning. This interesting proposition suggests studies relating situational characteristics to the quality of human reasoning.

Stanovich & West analyze individual differences with respect to response output (e.g., participants' numerical estimates). They do not analyze the underlying cognitive processes that led to the outputs; they thereby probably misclassify some non-normative responses as normative. Using base rate neglect and overconfidence as examples, I demonstrate the advantages of analyzing cognitive processes further.

The finding of a correlation between normative responses to judgment and reasoning questions and cognitive capacity measures (SAT score) suggests that the cause of the non-normative responses is computational in nature. This actually is consistent with the rational competence view. The implications of this finding for the adaptation view of cognition are discussed.

The target article focuses exclusively on System 2 and on reasoning rationality: the ability to reach valid conclusions from available information, as in the Wason task. The decision-theoretic concept of coherence rationality requires beliefs to be consistent, even when they are assessed one at a time. Judgment heuristics belong to System 1, and help explain the incoherence of intuitive beliefs.