Conditions for the acceptance of deontic conditionals.

Recent psychological research has investigated how people assess the probability of an indicative conditional. Most people give the conditional probability of q given p as the probability of if p then q. Asking about the probability of an indicative conditional, one is in effect asking about its acceptability. But on what basis are deontic conditionals judged to be acceptable or unacceptable? Using a decision theoretic analysis, we argue that a deontic conditional, of the form if p then must q or if p then may q, will be judged acceptable to the extent that the p & q possibility is preferred to the p & not-q possibility. Two experiments are reported in which this prediction was upheld. There was also evidence that the pragmatic suitability of permission rules is partly determined by evaluations of the not-p & q possibility. Implications of these results for theories of deontic reasoning are discussed.

Contingency, causation, and adaptive inference.

In contingency judgment tasks involving 2 event types, individuals weight the a and b cells of a 2 x 2 contingency table more than the c and d cells. Some theorists have argued that they can provide normative justifications for this weighting and that the weighting reflects simple heuristics that are adaptive in the real world. The authors show that, to avoid error, individual judgments about real contingencies should be more subtle than these supposedly adaptive heuristics allow.

Frequency versus probability formats in statistical word problems.

Three experiments examined people's ability to incorporate base rate information when judging posterior probabilities. Specifically, we tested the (Cosmides, L., & Tooby, J. (1996). Are humans good intuitive statisticians after all? Rethinking some conclusions from the literature on judgement under uncertainty. Cognition, 58, 1-73) conclusion that people's reasoning appears to follow Bayesian principles when they are presented with information in a frequency format, but not when information is presented as one case probabilities. First, we found that frequency formats were not generally associated with better performance than probability formats unless they were presented in a manner which facilitated construction of a set inclusion mental model. Second, we demonstrated that the use of frequency information may promote biases in the weighting of information. When participants are asked to express their judgements in frequency rather than probability format, they were more likely to produce the base rate as their answer, ignoring diagnostic evidence.

The effect of self-referent material on the reasoning of people with delusions.

People with delusions have been shown to have both generalized (Huq, Garety & Hemsley, 1988) and content-specific biases in reasoning (Bentall, 1994). Our concern here was whether the hastiness that has been found when people with delusions reason on relatively abstract tasks would be present on a more realistic task. A second concern was whether reasoning with salient or emotional material would increase the hastiness bias in people with delusions. Two versions of a probabilistic reasoning task were used to study the data gathering of people with delusions. The first version employed realistic but emotionally neutral material. People with delusions requested less evidence before making a decision than psychiatric and normal comparison groups. Therefore, the hastiness found previously with abstract materials was seen to generalize to a more realistic task. In the second version participants were required to reason with material that had an emotional content and may have been regarded as being personally meaningful. In this condition all groups reduced the amount of evidence requested before making a decision.

Normal and abnormal reasoning in people with delusions.

People who experience delusions have been found to request less information prior to making a decision than control participants on tasks that are unrelated to the theme of the delusion (Huq, Garety & Hemsley, 1988). Two studies investigated whether people with delusions have an absolute deficit in reasoning or a more specific data-gathering bias. In Expt 1, 12 people with delusions, 12 people with depression and 12 normal controls were shown the results of spinning a supposedly biased coin. The evidence provided varied in the number of heads to tails. In normal controls a high ratio of head to tails produces a high estimate that the coin is biased. In this experiment, where the evidence gathered was predetermined by the experimenter, all groups of participants were shown to reason in a similar way. Experiment 2 tested whether a difference would exist between the groups in conditions where participants were free to determine the amount of evidence seen, in contrast to when all of them viewed the same evidence. Two jars of beads in opposite but equal ratios (e.g. 85:15, 15:85) were shown to 15 people with delusions, 15 with depression and 15 normal controls. On the basis of beads being drawn one at a time, it was the participants' task to determine which jar they came from. When free to decide when they wished, people with delusions decided on the basis of less evidence than the other groups. However, as in Expt 1, the group with delusions did not differ when made to view the same amount of beads as other participants. Therefore, people with delusions have a data-gathering bias rather than a difficulty in employing the data in reasoning. This "jump to conclusions' bias generalized to a less discriminable ratio of beads (60:40), and was not a consequence of impulsiveness or memory deficit.

Reasoning, decision making and rationality.

It is argued that reasoning in the real world supports decision making and is aimed at the achievement of goals. A distinction is developed between two notions of rationality: rationality which is reasoning in such a way as to achieve one's goals--within cognitive constraints--and rationality which is reasoning by a process of logic. This dichotomy is related to the philosophical distinction between practical and theoretical reasoning. It is argued that logicality (rationality) does not provide a good basis for rationality and some psychological research on deductive reasoning is re-examined in this light. First, we review belief bias effects in syllogistic reasoning, and argue that the phenomena do not support the interpretations of irrationality that are often placed upon them. Second, we review and discuss recent studies of deontic reasoning in the Wason selection task, which demonstrate the decision making, and rational nature of reasoning in realistic contexts. The final section of the paper examines contemporary decision theory and shows how it fails, in comparable manner to formal logic, to provide an adequate model for assessing the rationality of human reasoning and decision making.

Social roles and utilities in reasoning with deontic conditionals.

A set of experiments is reported in which a new formulation of deontic thinking is tested. This is that people represent subjective utilities inherent in conforming to or violating deontic statements, along with the social dynamics of these statements. The experiments used Wason's selection task and tested people's understanding of conditional permission. In the first two experiments, familiar scenarios referring to family interactions were used. In the third, an imaginary business content was used. In both cases it was apparent that people's thinking depended on their representation of the utilities associated with the agent of a permission statement (the party who lays down the rule) and the actor (the party whose behaviour is its target). The results are discussed as favouring an explanation in terms of mental models, rather than the schema theories which have dominated this field hitherto.