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    There has been a recent upsurge of interest in individual differencesin reasoning which has been well summarised by Stanovich & West(2000). The reason for this is that certain types of individualdifferences can shed light on unresolved theoretical debates withinthe psychology of thinking and reasoning.

    One of the major debates concerns human rationality: the extent towhich humans are capable of rational, logical thought. There arenumerous instances of people giving non-rational answers toreasoning problems and the question arises as to whether thisdemonstrates lack of rationality or whether there are alternativeexplanations. It has been claimed by some that the incorrectnormative model has been applied, and that humans are rational butnot in the sense of adhering to the tenets of logic. One line ofevidence which goes against this is the well-established finding thatpeople higher in intelligence are more likely to produce the logicallycorrect answer to reasoning problems. This seems to indicate that thecorrect normative model has been applied (Stanovich & West, 2000).

    A related issue concerns the existence of two different types ofthinking. It has been claimed by many that there are two ways ofthinking, one slow, conscious, rational and controlled, the other fast,unconscious, experiential and automated. Unfortunately these aregiven different names by different people and are also defined indifferent ways (see Newstead, 2000). Nor is there any accepted wayof determining which type of thinking is being used or of determininghow they relate to each other. Once again, individual differences maythrow light on this. Recently-developed tests such as the Rational-Experiential Inventory (REI) (Pacini & Epstein, 1999) have thepotential to measure the extent to which a person relies on rationalthought or prior experience. Investigating how such thinking stylesrelate to reasoning performance can indicate which mode of thinkinghas been applied. Previous research, principally on statisticalreasoning, has indicated that rationality as measured by the REI,which is essentially a measure of the willingness to engage in logicalthinking, is related to accuracy on these problems. On the other handexperientiality, which is the tendency to use intuition and to rely onprior experience, is related to susceptibility to biases.

  • Objectives

    There were a number of objectives in the present researchprogramme.

    To determine the reliability and usefulness of the REI. This measurehas been developed and used only in the USA, and an importantpreliminary to the present research was to establish whether thescales are robust and suitable for use on a UK sample.

    To replicate the correlation between intelligence and performance ondeductive reasoning tasks such as syllogistic reasoning and theWason Selection Task. In addition, the aim was to look at therelationship between intelligence and statistical reasoning tasks, andto extend this to other reasoning tasks not previously used.

    To investigate the relationship between thinking styles andperformance on reasoning tasks. The initial interest was primarily inrationality and experientiality as measured by the REI. It waspredicted that rationality would tend to correlate with overallaccuracy and experientiality with biases in responding. Our researchalso went beyond these measures of thinking style and also used theThinking Disposition Composite, the Approaches to StudyingInventory, and a range of measures designed to investigate the abilityto produce different representations of the information presented.

    It should be noted that the objectives of the present research were tosome extent modified as the research progressed in order to takeaccount of novel findings which emerged. In particular, the firstseries of experiments produced results which were completelyunexpected and flew in the face of previous experimental findings.Rather than blindly following our original plan of research, whichwas in any case substantially undermined by these anomalousfindings, we were inevitably side-tracked into investigating thesource of the discrepancies. This led to us carrying out extendedstudies using a much wider range of individual differences measuresthan anticipated and a much wider selection of deductive reasoningtasks.

    This search for an explanation of our findings was ultimatelysuccessful and led to novel empirical findings which haveconsiderable theoretical importance. However, it meant that we hadless time to spend on the latter part of our initial project proposal,

  • with the result that we did not, as planned, investigate individualdifferences in statistical reasoning.

    Methods and Results

    In total eight studies were conducted, three more than were outlinedin the original proposal. The method and results for each of these willbe outlined in turn.

    Study 1

    In Study 1 we validated the REI using a postal sample of 128members of the general population and 98 undergraduate students.For the student sample we also administered a test of generalintelligence. The internal and test-retest reliabilities of all the REIscales were high and the factor structure was exactly as expected.Consistent with previous findings, men scored higher than women onrationality whereas women scored higher than men on experientiality.These scales were unrelated to general intelligence.

    This study confirms that the REI is a scale with good psychometricproperties and one which could be used effectively on a UK sample.

    Study 2

    Study 2 examined the relationship between the REI, intelligence, andperformance on abstract and realistic versions of the Wason selection task.This task is probably the most widely used in the psychology of reasoning.In the abstract version people are given four cards which have a letter onone side and a number on the other. They can see the top side of the cards,for example A, D, 4 and 7, and are asked which cards they need to turnover to determine whether the rule If there is an A on one side of the cardthen there is a 4 on the other side is true. The correct answer is to turnover the A and 7 cards, the latter because it would disprove the rule ifthere was an A on the other side. However, the great majority of peoplegive the wrong answer, saying it is necessary only to turn over the A and 4cards. Certain realistic versions of the task, e.g. using the rule If a personis drinking beer they are over 18, have been shown to produce massivefacilitation (Griggs & Cox, 1982).

    It was expected that there would be a correlation between intelligence andperformance on abstract selection tasks since this is the typical finding inprevious research (e.g. Valentine, 1975; Stanovich & West, 1998a). It was

  • also expected that the measure of experientiality would correlate withperformance on realistic tasks since the correct response has been assumedto derive in part from the use of prior experience. No prediction was madeconcerning correlations with rationality.

    The 98 participants, were given the 40-item version of the REI(Pacini & Epstein, 1999), and Part 1 of the AH5 intelligence test(Heim, 1968). Four selection tasks were used, two abstract and twodeontic. The abstract problems were the original letter-numberversion (If a card has an A on its letter side then it has 4 on itsnumber side) and a destination version (If Glasgow is on one side ofthe ticket, then train is on the other side of the ticket). Theinstructions asked participants to indicate which of the four cards(e.g., A, K, 4, and 5 in the first example) needed to be turned over totest whether the rule was true or false. The two realistic tasks were ananglicised version of the Sears problem in which Sears becameDebenhams (Any sale over 30 must be approved by the manager,Mr Jones) and the drinking age problem (If a person is drinkingbeer then the person must be over 18 years of age). The instructionsasked participants to choose the cards they thought needed to beturned over in order to test whether the rule was being violated.

    There was the usual facilitation of realistic over abstract selectiontasks. However, the correlations between the selection tasks and theindividual differences measures were quite unexpected. In contrast toprevious findings, there was no correlation between intelligence andperformance on abstract tasks. However, there was a significantcorrelation between intelligence and the realistic problems. Thesesurprising findings challenge the received wisdom in this area.

    Neither of the REI scales correlated with performance on theselection tasks. We had predicted that experientiality would correlatewith accuracy on the deontic (realistic) tasks.

    One possible explanation for the discrepancy with previous findingsis that we had used an inappropriate measure of intelligence or thatthe measures of deductive reasoning were unreliable or inaccurate insome way. In order to check on this, we decided to bring back asmany of the 98 participants as we could to repeat and extend thetesting we had carried out.

    Study 3

  • In this study, 44 of the participants used in Study 2 were given theREI and the same selection tasks as before. In addition they weregiven a further measure of ability, the AH4 Parts 1 and 2 (Heim,1967). Letter distance estimation, a measure of working memory, wasalso used. Sixteen syllogistic reasoning problems were used whichlead to conclusions which were either valid or invalid and eitherbelievable or unbelievable. This is an example of a valid, believablesyllogism:

    No dogs are unhappySome mammals are unhappyTherefore, Some mammals are not dogs

    The short form of the Approaches to Studying Inventory was used(Newstead, 1992) which measures three approaches to studying. Thiswas included mainly for exploratory purposes.

    The various individual differences measures were found to begenerally reliable. However, the selection tasks were less reliable,giving low test-retest scores. Nevertheless, the classic facilitation ofrealistic over abstract tasks was once again obtained.

    There was again no correlation between intelligence and performanceon abstract selection tasks; in fact the correlations were(nonsignificantly) negative. As in Study 2 there were significantcorrelations between intelligence and performance on realisticselection tasks. There were no significant correlations between eitherexperientiality or rationality and performance on any of the selectiontasks. The unusual and completely unexpected pattern of findings onthe selection task observed in Study 2 was thus confirmed in Study 3.

    With the syllogisms, the classic findings were broadly confirmed.There was a main effect of logic, with participants endorsing morevalid than invalid conclusions. They also endorsed more believablethan unbelievable conclusions, though this effect was not significant,and there was an interaction between these two variables.

    The main interest is in how these variables relate to measures ofindividual differences. Intelligence was significantly negativelyrelated to belief; in other words, people with higher levels ofintelligence were less likely to respond simply in line with thebelievability or otherwise of the conclusion. The correlation betweenintelligence and logic was not significant overall, but on thosesyllogisms where belief and logic were in conflict the correlationachieved significance. Participants of higher ability are thus able to

  • resist the belief-based response and provide a solution consistent withthe logical requirements of the task. Rationality was related to neitherbelief nor logic, but experientiality showed a negative relationshipwith logic. The findings on syllogistic reasoning were thus similar towhat has been obtained in previous research.

    Although Study 3 replicated the findings of Experiment 1, the samplesize was relatively small. The next study used a larger number ofparticipants and additional tasks.

    Study 4

    This study used 152 participants and similar materials to Study 3. TheAH4 and REI were used, and the same 16 syllogisms. Eight selectiontasks were used, four abstract and four realistic. The ASI and theletter distance tasks were dropped, but a further measure of thinkingstyle, the Thinking Dispositions Composite (Stanovich and West,1998b) was included. It was hoped that this might provide a measurerelated to intuitive thinking, though in actuality it produced fewresults of interest.

    There were no significant correlations between selection taskperformance and any of the individual differences measures. Theunexpected positive correlation between intelligence and deontictasks was not significant in the present study. However, it waspositive and on two of the tasks (the two we had used in Experiments1 and 2), the correlation was significant. There was no overallcorrelation between intelligence and abstract selection tasks thoughfor the first time in this series of studies there was such a correlationon one of the tasks, the original letter-number version.

    On the syllogisms, intellectual ability correlated with logic scores andwith scores on conflict problems (i.e., those where belief and logicpoint to different conclusions). In addition, rationality (as measuredby the REI) correlated positively with performance on nonconflictproblems. Experientiality did not correlate with any aspect ofreasoning performance.

    Thus these results broadly confirmed the findings of the earlierexperiments. The curious correlation between ability andperformance on realistic selection tasks was found once again, thoughonly on the two selection tasks used in earlier studies. In thisexperiment, for the first time, we observed a link between ability and

  • abstract tasks, though this was observed on only one problem, thestandard letter-number task. We were still unsure as to why ourresults were out of line with previous research and decided to carryout another study, this time on sixth form school students.

    Study 5

    Participants were 126 secondary school children aged between 16 and18. The REI and AH4 were used and also an additional measure ofintellectual ability, Ravens Progressive Matrices (Raven, 1983).Eight selection tasks were used, four abstract and four realistic, andthe syllogisms task as presented in Studies 3 and 4. AH4 and RavensMatrices scores correlated significantly and hence a compositemeasure of intellectual ability was used.

    On the selection tasks, there was a significant but much smaller thanusual facilitation on the realistic tasks. The ability compositecorrelated with only one of the four abstract tasks and rationalitycorrelated significantly, but negatively, with one of the other abstractproblems. In contrast to the previous experiments there was noevidence that realistic tasks correlated with ability.

    The pattern of results for the syllogisms was somewhat different tothat normally observed. There was a smaller than usual effect ofvalidity but a very large effect of belief, and the interaction betweenthese factors did not app...


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