Are people with schizophrenia more logical than healthy volunteers?

We tested a phenomenological hypothesis about theoretical andpractical rationality in people with schizophrenia. This hypothesisstates that in schizophrenia there is an enhancement of theoreticalrationality. Our case–control experiment supported thishypothesis. Philosophical models of rationality that prioritisetheoretical over practical rationality may thereby apply morein schizophrenic than in healthy states. The study is an exampleof how experimental psychopathology can illuminate areas of philosophical dispute that are difficult to settle by thoughtalone.

INTRODUCTION

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INTRODUCTION

There has been renewed interest in the phenomenological traditionin psychiatry (Owen & Harland, 2007) which offers a rangeof hypotheses about different psychopathological states. Onenotable hypothesis is that schizophrenia is an impairment ofcommonsense knowing (practical reason), with a preservation – or even accentuation – of systematic cognition(theoretical reason). This concept was made famous by the psychiatristMinkowski (1927) and can even be found in the anthropologicalwritings of Kant (1798) but has never been experimentallytested. Previous experimental work on rationality in schizophreniahas aimed to explain delusions in schizophrenia in terms of impairments of formal reasoning (Garety & Hemsley, 1995;Kemp et al, 1997). Results do not generally confirm this model (Cutting, 1997).

In line with the phenomenological hypothesis, we tested whethertasks that are correct from a theoretical (or formal logical)point of view but depart from practical knowledge (common sense)would be performed better by people with schizophrenia thanby healthy controls.

METHOD

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METHOD

Most philosophers conceptualise theoretical rationality as formallogical rationality for which deductive logic is held as theparadigm. Practical rationality or ‘common sense’,however, has been more difficult to conceptualise. It is generallytaken to denote non-formal rationality – a form of knowingthat provides the background assumptions about the world thatare the basis of shared human practice. It is tacit knowledgewithin a culture, and includes such things as the pre-theoreticalknowledge that the sun rises in the east or that hospitalsare buildings. Such knowledge is presupposed and used in everydaypractice, and as such becomes something that is separate fromtheoretical knowledge. The concept of common sense is that thereis a form of rationality that is independent of theoreticalrationality. The experiment we report assumes these two conceptsand takes them to be fundamentally different.

We operationalised theoretical reasoning using syllogisms thatwere deductively valid or invalid, and common sense using syllogisticcontent that strongly conformed to or departed from practicalknowledge. Two types of syllogism were constructed, in eachof which there was a conflict between deductive truth and commonsensetruth. The first type was non-commonsense syllogisms that werevalid (labelled ‘NCS’), for example:

‘all buildings speak loudly;
a hospital does not speakloudly;
therefore, a hospital is not a building.’

The second type comprised commonsense syllogisms that were invalid (labelled ‘CS’), for example:

‘if the sun rises, then the sun is in the east;
the sunis in the east;
therefore, the sun rises.’

Participants were asked by the interviewer (G.O.) to acceptthe first two sentences of each syllogism as true and thento decide on the truth or falsity of the third sentence. Theywere told that this rule applied to all the problems and wereasked to state it repeatedly until it was clear that they understoodit. All participants read the problems aloud. Syllogisms were scored as correct if they were answered logically.

To be more certain that our syllogisms did generate subjectiveconflict between a logical and a commonsense interpretationin healthy people, we had previously conducted an independentpilot study in which we tested 21 healthy individuals. Verbalreports confirmed the conflict between logical and commonsenseinterpretations. We discarded three syllogisms that accruedhigh scores on the basis that their commonsense content wastoo weak, leaving eight NCS syllogisms and seven CS syllogismsfor inclusion in the study reported here.

Ethical approval for the study was gained and all participantsgave informed consent. People diagnosed with schizophreniausing standardised criteria (DSM–IV; American Psychiatric Association, 1994)and healthy controls were asked to solve the syllogisms ina case–control design. Patients were selected from twoinner-London psychiatric hospitals; the sources were two general in-patient wards and the out-patient and in-patient facilitiesof a single service specialising in schizophrenia. All participantingpatients were taking antipsychotic medication. The controlgroup was selected from a wide variety of informal sources,including acquaintances, porters and staff at several hospitals,and advertisement. Exclusion criteria for both groups were age outside the range 18–65 years; premorbid IQ, estimatedusing the National Adult Reading Test (Nelson, 1994), outsidethe range 75–125 (as at extreme values this measure isa poor guide to full-scale IQ (Russell et al, 2000)); Englishnot native language; other neurological or psychiatric disorderor substance misuse. Medical records were reviewed for allpatients and a clinical interview was conducted by a psychiatrist(G.O.) to ensure that criteria were met. Of the 22 patientsapproached, two were excluded because of elicited historiesof epilepsy or heavy substance misuse and three because ofNART IQ score <75. Of the 21 potential control group members,one was excluded because aged >65 years and one because ofIQ score >125.

Our primary measures were number of syllogisms correct as atotal and as subsets according to type (NCS or CS). Potentialconfounding factors were considered to be IQ, age, gender andyears of education.

All t-tests performed were two-tailed with equal variance not assumed. Using percentage logically correct as the dependentvariable, we performed an inter-individual factorial analysisof variance testing for main effects by group (schizophreniav. control) and syllogism type (NCS v. CS) and their interaction.Our hypothesis was that the schizophrenia group would outperformthe control group.

RESULTS

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RESULTS

Groups were well matched, with 17 patients and 19 controls.There was no significant difference between the groups in premorbidIQ (t=–0.87, P=0.4), age (t=1.25, P=0.22) or years ofeducation (t=–0.06, P=0.96). About half (53%) of thecontrol group were men, compared with 65% of the schizophreniagroup.

Table 1 shows the group statistics. As predicted there wasa highly significant main effect by group (F_(1,68)=8.002, P=0.006),with patients outperforming controls. There was also a maineffect by syllogism type (F_(1,68)=52.916; P<0.001), butno interaction of syllogism type by group (F_(1,68)=0.157, P=0.69).The main effect by syllogism type showed that both groups scoredbetter on the NCS syllogism type than on the CS syllogism type.We take this to be the well-replicated ‘belief bias’effect (Evans, 2002), i.e. that logic has a larger effecton unbelievable (NCS) than on believable (CS) conclusions.

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Table 1 Logical responses to syllogisms by patients with schizophrenia compared with a healthy control group

In exploratory analysis of the group difference, the effectsize using the Cohen’s d statistic was 0.82 (large) forthe CS syllogism type and 0.54 (medium) for the NCS syllogismtype. Similarly, comparisons of means showed significance forthe CS syllogism type (t=–2.37, P= 0.026) but not forthe NCS type (t=–1.65, P=0.11). This suggests that theremight be an underlying interaction between syllogism type andgroup, with the CS syllogism type (commonsense reasoning) accountingfor most of the group difference, and that our failure to findit was due to inadequate statistical power.

DISCUSSION

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DISCUSSION

Our main results show that under conditions where common senseand logic conflict, people with schizophrenia reason more logicallythan healthy individuals. On a straightforward interpretationthis is either because people with schizophrenia are betterat logic or because they are worse at common sense. We presentsome exploratory evidence that it is because they are worse at common sense, but the question remains open.

A few limitations must be mentioned. The number of participantswas small, experimental designs using philosophical conceptsare novel and case–control studies cannot control forunknown confounding factors. For example, our stimuli did notallow for correct rejections of non-commonsense syllogismsor correct acceptance of commonsense syllogisms.

The results are intriguing because they shed light on reasoningin schizophrenia but also have significance beyond schizophreniaresearch. They suggest that in situations where commonsenseknowledge is at stake, formal norms of rationality are violatedby people with schizophrenia to a lesser extent than by healthyindividuals. People with schizophrenia seem to have a biastowards theoretical rationality over and above practical rationality.It is an ongoing dispute within philosophy of science whether,as a matter of principle, theoretical reason has priority overpractical reason or vice versa (Thagard, 2004). Given that schizophrenia is at its core a pathological state of thinking,our results suggest that concepts of rationality that prioritisetheoretical reason over and above practical reason might applymore accurately in a pathological example of human thinkingthan in a healthy one. This is an example of how experimentalpsychopathology can shed light on fundamental philosophical debates that have not been settled by argument alone.

REFERENCES

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