J. St B. T. Evans

Dual processes in reasoning

Abstract Previous results have shown that the introduction of negatives into the sentences used in a deductive problem affected behavior in a systematic way which was independent of the logical structure of the problem. In the present investigation, the subjects were asked to justify their responses when reasoning about such sentences. In accordance with previous results, the responses were dominated by the terms in the sentences regardless of whether they were negated. However, the justifications did vary when negatives were introduced in accordance with the logical consequences of the responses. The interpretation of these justifications as causes of behavior seemed implausible. It was suggested that they were rationalizations, or that there was at least some form of dual processing between behavior and conscious thought.

Debiasing by instruction: The case of belief bias

Abstract The study is concerned with the question of whether robust biases in reasoning can be reduced or eliminated by verbal instruction in principles of reasoning. Three experiments are reported in which the effect of instructions upon the belief bias effect in syllogistic reasoning is investigated. Belief bias is most clearly marked by a tendency for subjects to accept invalid conclusions which are a priori believable. Experiment 1 attempted to replicate and extend an experiment reported by Newstead, Pollard, Evans and Allen (1992). In contrast with their experiment, it was found that belief bias was maintained despite the use of augmented instructions which emphasised the principle of logical necessity. Experiment 2 provided an exact replication of the augmented instructions condition of Newstead et al., including the presence of problems with belief-neutral conclusions. Once again, significant effects of conclusion believability were found. A third experiment examined the use of elaborated instructi...

Language and reasoning: a study of temporal factors☆

The paper is concerned with the testing of psycholinguistic hypotheses by the use of deductive reasoning tasks. After reviewing some of the problems of interpretation which have arisen with particular reference to conditional rules, an experiment is presented which measures comprehension and verification latencies in addition to response frequencies in a truth table evaluation task. The experiment tests a psycholinguistic hypothesis concerning the different usage of the logically equivalent forms of sentence: If p then q and p only if q with respect to the temporal order of the events p and q. It is proposed that the former sentence is more natural when the event p precedes the event q in time, and the latter more natural when the opposite temporal relation holds. Although significant support is found for the hypothesis in the analysis of the latency data, it is only distinguished from an alternative explanation by detailed analysis of response frequencies, thus indicating the general usefulness of the paradigm adopted.

Hits and misses: Kirby on the selection task

Kirby (1994) has helpfully applied the terminology of signal detection theory to Watson’s selection task. We agree with him that it is necessary to bring out the decision theoretic aspects of the selection task (Evans, Over & Manktelow, 1993), and Kirby makes an important contribution by doing this so fully for an abstract version of the task. However, there are problems with both the design of Kirby’s experiments and the interpretation which he places upon them. Kirby discusses his abstract selection tasks in a rather abstract way, but his points can be made with more intuitive examples. More importantly, these points can be related to fundamental issues in the philosophy of science, particularly those stressed by Bayesian thinkers. Using a standard intuitive example, we can bring out the general interest of Kirby’s work as well as some of its difficulties. (See Howson & Urbach, 1989, for a good discussion in the philosophy of science of essentially the example we shall use. Wetherick, 1993, notes the relevance of this example to the selection task.) Consider the standard example in the philosophy of science, expressed in a conditional form: if it is a raven then it is black. Suppose we were given the task of finding out whether this statement, taken as one about all things, can be falsified. To do this, we would be much better advised to go out looking for ravens than for non-black things. It is true that we would have a falsifying case if we found a non-black thing which was a raven. But the set of ravens is small relative to that of non-ravens, and thus any non-black thing we find is almost bound to be a non-raven in any case. It is better to search through the relatively circumscribed set of ravens. Following Kirby’s lead, we can neatly describe the possible results of this search in the terminology of signal detection theory. For the task we were given, finding a raven which is non-black is a hit, but finding a raven which is

On interpreting reasoning data — A reply to Van Duyne

In a previous article (Evans, 1972a) it was argued that theorists had misinterpreted data in reasoning experiments by assuming that subjects were responding to the logical structure of the problems. Instead it was proposed that results should be interpreted in relation to two kinds of ‘non-logical’ variables, those which affect the interpretation of the sentences forming the propositions, and those which affect the reasoning operations involved. One such operational variable discussed was ‘matching bias’ or the tendency for subjects to select values named in logical rules for testing hypotheses, irrespective of the logical consequence of such selections. This has been demonstrated by manipulating the presence of negative components in conditional rules (Evans, 1972b, Evans & Lynch, 1973). Van Duyne (1973, 1974) in criticising my approach to these problems has concentrated on the problem of ‘matching bias’ as an explanatory concept. His criticisms may be summarized as follows: (i) The matching bias effect lacks empirical generality (ii) My psychological explanations of the effect are vacuous (iii) The postulation of a ‘non-logical’ matching bias is inconsistent with introspective protocols which show that “in these experiments subjects are trying to reason correctly” (Van Duyne, 1973, p. 240) In support of the explanatory power of the matching bias hypothesis it should be recalled that it is the only explanation of Wason’s well known ‘selection task’ (for detailed discussion see Wason & Johnson-Laird, 1972) to have been deduced from an entirely independent reasoning situation (Evans, 1972b) and subsequently predicted and observed on the selection task (Evans & Lynch 1973). It is true that matching bias does not generalize to problems with realistic materials, at least so far as the selection task is concerned, since in these circumstances subjects tend to produce the logically correct responses, (e.g., Wason & Shapiro, 1971, Johnson-Laird, P. Legrenzi &, M. Legrenzi, 1972), presumably because there is a strong semantic basis for interpretation. Van Duyne correctly states that the effect is also not observed on abstract tasks when disjunctive rules are used. This is rather puzzling but the introduction of negative components produces sentences which are so unnatural as to be almost impossible for subjects to interpret (see Evans 1972a). I do, however, have some data to show that the