Schizophrenia Bulletin Advance Access originally published online on February 16, 2005
Schizophrenia Bulletin 2005 31(1):105-116; doi:10.1093/schbul/sbi012
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Schizotypy and Conditional Reasoning
Lecturer in Psychology, University of Wales Institute Cardiff, Cardiff, Wales, U.K
Professor of Experimental Psychology, School of Psychology, Cardiff University, Cardiff, Wales, U.K
Consultant Clinical and Forensic Psychologist, South Wales Forensic Psychiatric Service at Caswell Clinic, Glanrhyd Hospital, South Glamorgan, Wales, U.K.; and Reader, School of Psychology, Cardiff University
Send reprint requests to Dr. J. Sellen, Centre for Psychology, School of Health and Social Sciences, University of Wales Institute Cardiff (UWIC), Llandaff Campus, Western Avenue, Cardiff, CF5 2YB, U.K.; e-mail: JSellen{at}uwic.ac.uk.
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Abstract |
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This study investigated the role of reasoning biases in delusion formation and maintenance. Reasoning judgments have been shown to be influenced by prior knowledge, beliefs, and experience—that is, information stored in semantic memory. It was hypothesized that high schizotypes would exhibit a "jump to conclusions" style of reasoning as a result of not using implicit information concerned with cause-effect relationships. Research into human reasoning has traditionally adopted logic as a normative framework to assess human reasoning. Conditional inference tasks are direct tests of logical performance, and we employed an established design that depends upon the reasoner's ability to access and use implicit information. In an effort to negate some of the difficulties of research with schizophrenia patients, schizotypy measures were employed in a normal population. The results confirmed that high scorers on one dimension of schizotypy (Impulsive Nonconformity) failed to take account of the number of counterexamples that characterized the cause-effect conditional statement. These observations supported previous research demonstrating a jump to conclusions style of reasoning that it has been suggested plays a role in the formation and maintenance of delusions. Furthermore, these findings suggest a possible link between semantic memory and reasoning biases.
Keywords: Reasoning / logic / overinclusive thinking / schizotypy / O-LIFE / data-gathering bias
There have been many attempts to understand and explain the symptoms of schizophrenia, in particular delusions, a key feature of this illness and some affective psychoses. Some researchers believe that reasoning abnormalities or biases may contribute to the formation and maintenance of delusions (Huq et al. 1988
; Garety 1991; Garety and Hemsley 1994; Garety and Freeman 1999). The research presented here investigates the hypothesis that high schizotypes may be impaired in seeking exceptions in a conditional inference task, the archetypal test of how people should reason. We first introduce the conditional inference task, then explain why we expect semantic memory deficits (also found in patients with schizophrenia) to influence reasoning. We then outline evidence for the role of reasoning deficits in the formation and maintenance of delusions. After that, we discuss some current debates concerning schizotypy and psychosis-proneness research that are relevant to this article because the participants who completed this study were from a normal population who completed the Oxford-Liverpool Inventory of Feelings and Experiences (O–LIFE) schizotypy measure (Mason et al. 1995).
The view that delusion formation and maintenance may be explained in terms of a "reasoning abnormality" has been put forward by Garety and colleagues (see Garety and Hemsley 1994 for a review). However, before any such explanation can be fully accepted, there must be a framework with which to understand "normal" human reasoning.
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Conditional Inference Task and Theoretical Approaches to Human Reasoning |
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 TopAbstract Conditional Inference Task and... Reasoning and Semantic MemoryRole of Reasoning Deficits...Schizophrenia, Schizotypy, and...MethodsResultsDiscussionAppendix. Conditional statements...References |
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Theoretical approaches to human reasoning have for many years accepted formal logic as providing a normative model of how people should reason. One of the most direct tests of logical performance are conditional inference tasks, which use the logical construct, "If ... then." In this task, participants assess inferences from conditional rules, "If p, then q." Two inferences are logically valid: modus ponens (MP), which takes the form "If p, then q," "p, therefore q"; and modus tollens (MT), which takes the form "If p, then q," "not-q, therefore not-p." The other two inferences that participants can endorse are fallacies according to the rules of logic: denying the antecedent (DA), which takes the form "If p, then q," "not-p, therefore not-q," and affirming the consequent (AC), which takes the form, "If p, then q," "q, therefore p." Performance is deemed logical if participants endorse MP and MT and reject DA and AC.
According to this normative model, the conclusion will follow even in the light of additional or conflicting information. For example, take the conditional "If Claire turned the key, then the car started." Given that Claire turns the key, the conclusion that the car starts must hold (according to a logicist model) even if the engine has been removed, or if there is no gasoline, or if the starter motor is broken. However, intuitively and empirically, we know that in everyday reasoning, people make use of prior knowledge and experience in drawing inferences. Using the example above, in the face of the engine having been removed, or having no gasoline, or having a broken starter motor, most people would revise their conclusion that the car would start as long as the key was turned.
Indeed, there is now considerable empirical evidence demonstrating that people's performance on reasoning tasks deviates from the logicist model, and much effort has gone into explaining why performance does not fit this framework (e.g., Manktelow and Over 1991, 1995; Oaksford and Chater 1994, 1998; Stanovich 1999). Performance on reasoning tasks has been shown to be influenced by the content of reasoning problems (e.g., Cheng and Holyoak 1985, 1989; Cosmides 1989; Oaksford and Chater 1994), the perspective a reasoner is cued to in a reasoning task (e.g., Manktelow and Over 1991, 1995; Gigerenzer and Hug 1992), individual differences in intellectual functioning (Stanovich and West 1998a, 1998b), additional or alternative explicit information (e.g., Byrne 1989; Stevenson and Over 1995), and implicit information embedded in causal relations (e.g., Cummins et al. 1991; Cummins 1995). For instance, take the conditional rule "If you turn the key, then the car starts." If one believes that there is no gasoline in the car, or that the battery is dead, for example, then one may be less likely to conclude that the car will start given that the key has been turned (MP); or that the key was not turned, on learning that the car had not started (MT). People become less willing to endorse the logical inferences when such exceptions are available (Byrne 1989; Cummins et al. 1991; Cummins 1995). This so-called suppression effect is a robust finding in the reasoning literature, with over 40 studies reporting similar effects (for reviews, see Evans et al. 1993; Manktelow 1999).
Oaksford and Chater (1998) suggest that when drawing inferences and reasoning, "people have access to representations of the world" (p. 11). These representations of prior knowledge influence our reasoning judgments. Such information is stored in our long-term knowledge base—that is, semantic memory (Oaksford and Chater 1998). So, the ability to store, access, and use information held in semantic memory plays a crucial role in everyday human reasoning judgments (Byrne 1989; Cummins et al. 1991; Thompson 1994; Cummins 1995; Oaksford and Chater 1998; Quinn and Markovits 1998). The evidence outlined above has shown that information provided, implied, or stored in semantic memory influences reasoning judgments, and this therefore demonstrates the inadequacy of logic as a normative model to explain human reasoning. Indeed, Oaksford and Chater (1998) suggest that employing the rules of logic to explain performance on conditional inference tasks is misguided.
By embedding cause-effect relations in conditional inference tasks, Cummins et al. (1991) and Cummins (1995) demonstrated that manipulation of exception information implicitly available in semantic memory increased rejection of the logical inferences (MP and MT), thereby increasing nonlogical responses. In their studies, exceptions are called disabling conditions. Disabling conditions are factors that may prevent an effect from occurring in the presence of a known cause. The greater the number of disabling conditions that characterize the cause-effect relationship, the more doubt is cast on the sufficiency of the cause to bring about the effect. For example, using the conditional rule "If you turn the key, then the car starts" admits many disabling conditions (e.g., no gasoline, dead battery) and therefore casts doubt on the car starting given that the key was turned. In the same studies, other information in the form of alternative causes was also manipulated. Alternative causes are causes other than that which is stated, that may bring about the same effect. When there were many alternative causes characterizing the causal relationship, it was found that endorsement of the fallacies (DA and AC) decreased. The greater the number of alternative causes implicitly available, the more doubt is cast on the necessity of the given cause to bring about the effect. For example, a rule such as "If you turn on the air conditioner, then you feel cool" admits many alternative causes that may bring about the same effect (e.g., the window may be open, it may be a cold day).
The results found in these two studies (Cummins et al. 1991; Cummins 1995) provide clear evidence that in reasoning, people do take alternatives and exceptions into consideration; indeed, this search for counterexamples may be a "general reasoning strategy" (Cummins et al. 1991, p. 280). The alternatives and exceptions in these studies were based on implicit prior knowledge stored in semantic memory. The general reasoning strategy suggested by Cummins et al. (1991) provides yet more evidence for a direct link between reasoning and semantic memory.
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Reasoning and Semantic Memory |
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The evidence that other factors, such as additional or alternative information, can influence reasoning may have important implications for research with schizophrenia patients (and those who suffer from delusions in other illness domains) for two main reasons. First, it has been shown that patients, as well as individuals high in delusional ideation, seek less information before reaching a decision; that is, they have a data-gathering bias. This bias leads to a "jump to conclusions" style of reasoning (Huq et al. 1988
; Garety and Hemsley 1994; John and Dodgson 1994; Dudley et al. 1997; Linney et al. 1998; Garety and Freeman 1999). If this is the case, then we might expect that patients would not carry out the general reasoning strategy of searching for counterexamples. Instead, they may jump to conclusions that normal participants would not draw. Second, evidence that additional or alternative information can influence reasoning is relevant to research on semantic memory deficits in schizophrenia. It has been argued that patients are prone to overinclusive thinking (Cameron 1939, 1945; Chapman and Chapman 1973; Chen et al. 1994). Chen et al. (1994) found that patients include exemplar items that are outside the boundaries of a category but are related to it (e.g., they include "airplane" in the category "bird"). This evidence suggests that information is not stored efficiently. If category boundaries are unclear, then a general reasoning strategy that depends on searching for information in semantic memory is likely to be prone to error.
We argue that these two cognitive features of schizophrenia patients, overinclusive thinking and a jump to conclusions style of reasoning, may be inherently connected. The observation that patients include more related (although incorrect) items within a given category may be understood in terms of ignoring exceptions, as schizophrenia patients seem to obey a general rule (e.g., "If it flies, it is a bird") but ignore the exceptions to it (e.g., unless it is an airplane, unless it is a bat). This is also consistent with the associative intrusions found in schizophrenia patients and their families, whereby verbal communication failures include the use of overinclusive, vague, or ambiguous words (Docherty et al. 1999). Patients produce related (but incorrect) items and appear unable to select the relevant from the irrelevant members of the semantic category in their thoughts or language. The evidence of overinclusive thinking and ignoring exceptions is consistent with the reasoning task reported here, which directly tests the ability to manipulate exception information. Garety and Freeman (1999) suggest that the jump to conclusions style of reasoning is not a function of a memory deficit. However, previous tasks, such as the probabilistic reasoning tasks adopted by Garety and colleagues, do not draw on semantic memory in the same way as this study's implicit conditional inference task does. We therefore suggest that the ability to accurately draw inferences depends not only on reasoning and data gathering but also on the reasoner's ability to successfully store and access prior knowledge in semantic memory. Thus, we view reasoning and semantic memory to be inherently linked (see also Oaksford and Chater 1998).
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Role of Reasoning Deficits in the Formation and Maintenance of Delusions |
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There are two main theoretical accounts of the etiology of delusions. First, Maher (1974
, 1988, 1992) argues that the cognitive apparatus remains intact and that delusions arise as a result of rational attempts to make sense of unusual perceptual experiences. Second, Garety and coworkers (see Garety and Hemsley 1994 for a review) argue that the formation and maintenance of delusions may be explained not by a global deficit in cognitive systems (e.g., Andreasen 1999) but by more specific deficits in reasoning strategies, and even more explicitly, biases in data gathering (Hemsley and Garety 1986; Huq et al. 1988; Garety 1991; Garety et al. 1991; Garety and Hemsley 1994; John and Dodgson 1994; Dudley et al. 1997; Young and Bentall 1997; Garety and Freeman 1999). We have adopted a similar position in this research.
Garety and Freeman (1999) suggest that a jump to conclusions style of reasoning represents a data-gathering bias that leads to early acceptance or rejection of hypotheses. Mujica-Parodi et al. (2001) suggest that delusions, given their resistance to change even in the face of contrary evidence, may arise from faulty reasoning as a result of drawing false conclusions from faulty a priori premises. Garety and colleagues make a clear link between reasoning errors and delusions by suggesting that inaccurate judgments based on insufficient information contribute to delusion formation and maintenance (Hemsley and Garety 1986; Garety 1991; Garety et al. 1991; Garety and Hemsley 1994; Garety and Freeman 1999).
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Schizophrenia, Schizotypy, and Psychosis-Proneness Debates |
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TopAbstractConditional Inference Task and...Reasoning and Semantic MemoryRole of Reasoning Deficits...Schizophrenia, Schizotypy, and...MethodsResultsDiscussionAppendix. Conditional statements...References |
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Research with schizophrenia patients often involves a number of difficulties. For example, possible confounds are created by pharmacological agents, the nature and severity of symptoms at the time of testing, the lack of motivation (as well as research fatigue), the inability or unwillingness to accurately report symptoms, institutionalization, and the frequent presence of a generalized decline in cognitive performance (Claridge 1988
). Schizotypy as a concept has proved useful in investigating some of the phenomena associated with schizophrenia and psychosis, as it obviates some of these methodological problems. There is now considerable evidence for the construct validity of schizotypy. High schizotypes resemble schizophrenia patients on a number of experimental paradigms (e.g., Baruch et al. 1988; Kidd and Powell 1993; Obiols et al. 1993; Lenzenweger and Korfine 1994; Williams 1995; Linney et al. 1998; Gray et al. 2002).
However, there are a number of issues fiercely debated in schizotypy research. First, the "clinical versus individual differences" debate concerns whether schizotypy is a continuous dimensional variation in proneness to psychosis occurring naturally in the general population, or a discontinuous genetic variation of proneness to disease (schizophrenia). The clinical view, put forward by Meehl (1962, 1990), suggests that a specific, inherited, neural defect ("schizotaxia") renders some people vulnerable to a latent liability for the development of various forms of schizophrenic illness, measurable along a "schizotypy continuum." For this model, schizotypy is discontinuous with normality (Lenzenweger and Korfine 1995; Tyrka et al. 1995). However, this is not to suggest that those vulnerable will decompensate to the full range of schizophrenic symptoms. Indeed, of the 10 percent of the population Meehl suggested had a "schizotypy taxon," half are thought to carry this liability to schizophrenia completely undetected across their life span (Lenzenweger and Korfine 1995). Given that half of the people thought to carry a schizotypy taxon may never manifest even mild symptoms of illness, much effort has gone into developing research methods to detect schizotypy (e.g., psychometric methods as well as genetic relatedness). This clinical view has been described by Claridge and Beech (1995) as "quasi-dimensional"—that is, it construes dimensionality as degrees of expression along a disease continuum. In contrast, Claridge and Beech argue that a "fully dimensional" approach to schizotypy is more appropriate. This view is often known as the individual differences approach. This model encompasses the disease continuum but takes as its starting point normality (or health); hence, schizotypy is viewed as continuous with normality (Claridge 1985, 1997; Chapman et al. 1995; Claridge et al. 1996). Supporting this approach, Claridge (1990) proposes that schizotypy is a dimension of personality, suggesting that there is a "tendency for characteristics of the psychotic states to be found, in mild degree, among healthy people" (p. 31). The most common method of measuring schizotypy in this approach is via psychometric questionnaire in the general population.
Another debate concerns whether psychosis and schizophrenia exist on a continuum, and if so, whether schizotypy is a measure of psychosis-proneness (Chapman et al. 1995; Claridge 1997) rather than a measure of predisposition to schizophrenia per se (Lenzenweger and Korfine 1995; Venables 1995). First, we summarize the arguments for a psychosis continuum, compared to a dichotomous view of psychosis and schizophrenia. We then outline the implications of this debate for schizotypy. Kraepelin (1919, cited in Claridge 1997) distinguished between an affective, manic-depressive form of mental illness (commonly known as bipolar affective disorder) and schizophrenia. This view is supported by classification systems such as DSM–IV and ICD–10, used by researchers and practitioners alike. However, some researchers have recently begun to suggest that this dichotomy may be fatuous (e.g., van Os et al. 2000; Johns and van Os 2001). Taylor (1992) points out that "schizophrenia and affective disorder are not distinct conditions but are, in fact, opposite ends of a continuum on which schizophrenia is the severest form and schizoaffective disorder is intermediate (Crow, 1990)" (p. 22). Taylor (1992) conducted a review of literature examining family, twin, and adoption data and found that schizophrenia and affective illness do co-occur in some families. Kendler et al. (1985) found that the relatives of schizophrenia patients (compared to relatives of control patients) had a significantly higher risk for schizoaffective disorder and a risk for bipolar disorder that approached significance. Similarly, Kendler et al. (1986) found a higher risk for schizophrenia in first degree relatives of patients with psychotic affective disorder. It has therefore been suggested (e.g., Crow 1986, 1991; Kendell 1991; Taylor 1992; Van Os et al. 2000; Johns and Van Os 2001) that psychosis exists on a continuum from normal through affective disorders to schizophrenia. Claridge (1997) adopts a similar view, but instead of a psychosis continuum, he draws upon the unitary (known as Einheitpsychose) theory of psychosis and argues that there is a single psychotic state of which the illnesses of schizophrenia and manic-depressive psychosis are different expressions. While there is considerable debate regarding the existence of a dichotomy, a single psychotic state with differing expressions, or a psychosis continuum (see Taylor 1992 for a full review), we adopted the latter position in this research. Although we will maintain this view, to conform with convention, we will use the narrower construct term of schizotypy throughout.
These views on whether schizophrenia and affective psychoses are unitary (part of a continuum or different expressions of a single "illness") or dichotomous are evident in the debates over whether three or four factors accurately define the structure of schizotypy (see Venables 1995 and Mason et al. 1997 for reviews). Most questionnaire measures of schizotypy reflect the three symptom clusters of schizophrenia (Liddle 1987): positive symptoms of hallucinations and delusions, negative symptoms of withdrawal and flat affect, and thought disorder or disorganization (e.g., Venables and Bailes 1994; see also Bergman et al. 2000; Reynolds et al. 2000; Venables and Rector 2000; Vollema and Hoijtink 2000 for current arguments for a three-factor model of schizotypy). As such, these measures are assumed to gauge some kind of proneness to schizophrenia (Lenzenweger and Korfine 1995; Venables 1995). However, some schizotypy measures include a fourth factor, which tends to reflect an impulsive/asocial component (e.g., Muntaner et al. 1988; Bentall et al. 1989; Kendler and Hewitt 1992; Venables and Bailes 1994; Claridge et al. 1996). Mason et al. (1997) suggest that this fourth factor is akin to the antisocial behavior of some psychotics. They also argue that it is related to "aspects of bipolar affective disorder and borderline personality disorder" (p. 33). This supports the notion of a psychosis-proneness continuum, where an Impulsive Nonconformity factor measures the more affective aspects of psychosis.
The psychosis-proneness approach that is adopted in this article is supported by four-factor schizotypy measures such as the Combined Schizotypal Traits Questionnaire (CSTQ, Bentall et al. 1989). The CSTQ, a 420-item self-report questionnaire, was constructed from 18 existing schizotypy and personality scales (see Claridge et al. 1996 for full details) in an attempt to identify the factor structure of schizotypy. Bentall et al. (1989) extended the previous three-factor model by revealing a factor that seemed to reflect a disinhibited or asocial schizotypy. Claridge et al. (1996) extended these findings by increasing the number of participants who completed the CSTQ from 180 to 1,095. They also carried out principal components analysis and confirmed the existence of a four-factor model of schizotypy. Claridge et al. (1996) argued that this model represents a psychosis continuum, highlighting the evidence that some scales not designed to measure schizotypy have high loadings on schizotypy factors. The STB scale (Borderline Personality Scale, Claridge and Broks 1984) was found to have a high loading on the disorganization factor of schizotypy. The STB scale was designed to measure characteristics more akin to borderline than schizotypal personality disorder, and as such, Claridge et al. (1996) argued that this is "closer to affective than to schizophrenic psychoses" (p. 112). Another scale not designed to measure schizotypy is the Hypomanic Personality Scale (Eckblad and Chapman 1983), which was found to have a high loading on the positive factor of schizotypy in the Claridge et al. and Bentall et al. studies. Furthermore, these two scales, as well as Eysenck's P-scale, were found to have high loadings on the Impulsive Nonconformity factor. It is the evidence for the multidimensional nature of schizotypy, where factors not designed to measure schizotypy have high loadings on positive and disorganized schizotypy factors, that has led to the argument for a psychosis continuum model that incorporates affective psychosis as well as schizophrenic psychosis (i.e., normal through affective, and schizoaffective to schizophrenia) rather than the narrower schizotypy continuum (i.e., schizotypy factors are viewed as akin to the symptoms of schizophrenia)
One of the major drawbacks of the CSTQ is the length (420 items) and the repetitive nature of the questions. Mason et al. (1995) therefore developed a new scale, the O–LIFE, which is derived from the CSTQ and maintains the four-factor model. This four-factor model of psychosis-proneness is the model that is adopted in this article. By convention, however, these measures of psychosis-proneness continue to be termed schizotypy measures, and in maintaining convention, we will also continue to use the term schizotypy.
We showed earlier that nonlogical performance on a conditional inference task arises when there are a number of counterexamples available (Cummins et al. 1991; Cummins 1995). Following research that has shown that patients demonstrate overinclusive thinking (e.g., Chen et al. 1994), we hypothesized that high schizotypes would be unable to carry out the general reasoning strategy of searching for implicitly available counterexamples (because of a process of ignoring exceptions to a general rule), leading to a jump to conclusions style of reasoning. We tested this hypothesis by employing the O–LIFE schizotypy measure with a normal population on the Cummins (1995) causal conditional inference task.
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Methods |
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Participants. Sixty-four psychology students (46 females, 18 males) from Cardiff University participated in this study, receiving either course credit or payment. Undergraduate students are obliged to complete 10 hours as research participants each semester. No in-depth description of the study was provided beforehand, to avoid any recruitment bias resulting from the nature of the study. Students were excluded from participating if their first language was not English and if they had received any training in formal logic. Ages ranged from 18 to 46 (mean = 21.67, standard deviation [SD] = 4.81).
Schizotypy Measure.
All participants completed the O–LIFE (Mason et al. 1995), which includes four subscales, Unusual Experiences (UnExp), which contains items consistent with the positive symptoms of psychosis (e.g., delusions and hallucinations); Impulsive Nonconformity (ImpNon), which refers to disinhibited, violent, self-abusive, and reckless behaviors; Cognitive Disorganization (CogDis), which includes items that measure attention, concentration, social anxiety, and decision making and is thought to be consistent with thought disorder in schizophrenia; and Introvertive Anhedonia (IntAn), which includes items that describe a dislike of physical and emotional intimacy, emphasizes independence and solitude, and reflects the negative symptoms of schizophrenia.
Materials.
Sixteen causal conditional statements (Cummins 1995; see appendix) that had been pretested and manipulated for the number of counterexamples were used in this study. The statements had been pretested (see Cummins 1995) for the number of alternative causes and disabling conditions and varied between "many" and "few" for each condition. Each statement was embedded within the four inference types (i.e., MP, DA, AC, and MT), providing 64 statements. The 64 statements were ordered such that no conditional statement followed one with the same origin (i.e., statement 1 followed by statement 1) or with the same combination of alternative causes and disabling conditions (e.g., few alternative causes, many disabling conditions). Also, no inference type (i.e., MP, DA, AC, and MT) was presented consecutively.
Procedure.
Each participant was tested individually. Instructions and stimuli were presented on an Apple Macintosh computer, using PsyScope software (Cohen et al. 1993). The participants completed 64 trials. For each trial, the statement, a fact, a conclusion, and a rating scale appeared simultaneously. For example, for the statement "If Claire turned on the air conditioner, then she felt cool," the fact in the MP condition would be "Claire turned on the air conditioner" and the conclusion would follow "Therefore Claire felt cool." Participants were asked to rate how sure they were that the conclusion could be drawn, on a scale of –3 (very sure that conclusion cannot be drawn) to +3 (very sure that conclusion can be drawn). This they did by pressing the appropriately marked key on a keyboard in front of them. After their decision was recorded, there was an 800-millisecond delay. An exclamation mark then appeared; it acted as a prompt for participants to press the spacebar when they were ready to continue to the next trial. There was a 300-millisecond delay between the spacebar press and the commencement of the next trial. In the instructions participants were told to read the material carefully before making a decision. They were also advised that there was no time limit but that they would be unable to return to earlier trials, so they should consider their responses carefully. Participant identification numbers, reaction times, and rating decisions were recorded by computer.
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Results |
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Schizotypy Scores. Analysis was carried out on the four factors of the O–LIFE. The means and SDs for the normative data for each factor are shown in table 1, together with those obtained in our sample. The overall means for UnExp and IntAn were significantly lower in our sample compared to the normative data. The t test values were as follows: UnExp t(570) = 3.91, p < 0.01; IntAn, t(570) = 3.00, p < 0.01.
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The correlations between the O–LIFE factors in this study compared to the normative data can be seen in table 2. The most obvious difference is between ImpNon and IntAn and ImpNon and CogDis, where our correlations failed to reach the significance levels seen in the normative data. These differences in significance may, however, be due to differences in power relating to differences in sample size. Fisher's transformation confirmed that there were no significant differences between the magnitude of correlations from the normative data and those obtained in this study.
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As reported in the Methods, there were more female than male study participants. The range, mean score, and SD on each schizotypy factor for males and females can be seen in table 3. Unpaired t tests confirmed that there were no differences between gender on any of the four schizotypy factors.
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Replication of Cummins (1995).
We successfully replicated the Cummins (1995) experiment. We found that the number of alternative causes and disabling conditions that characterize the causal relationship does significantly influence the acceptance of the inferences, regardless of the logical structure of the arguments.
A 2 x 2 x 4 repeated measures analysis of variance with alternative causes (many, few), disabling conditions (many, few), and inference type (MP, DA, AC, MT) as factors and acceptance rating as the dependent variable was performed. We found a significant effect of number of alternative causes, whereby inferences that had few alternative causes had higher acceptance ratings (mean = 1.54, SD = 1.36) than inferences with many alternative causes (mean = 0.61, SD = 1.52), F(1,63) = 176.38, p < 0.0001. We also found a significant main effect of disabling conditions. Similarly, inferences that had few disabling conditions had higher acceptance ratings (mean = 1.19, SD = 1.51) than inferences with many disabling conditions (mean = 0.96, SD = 1.51), F(1,63) = 18.07, p < 0.0001. No significant interaction was found between alternative causes, disabling conditions, and inference type, F(3,189) = 1.28, p = 0.28. The mean acceptance ratings for all inferences with many and few counterexamples (alternative causes and disabling conditions) are shown in table 4. As Cummins (1995) found, only the DA and AC inferences were influenced by the number of alternative causes characterizing the causal relationship, while only MP and MT inferences were influenced by the number of disabling conditions.
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Reaction Time Data. Reaction time data were explored for all analyses. No significant differences were found between high and low groups on any of the schizotypy factors, for any of the analyses.
Schizotypy and Exceptions. To examine the effects of schizotypy on disabling conditions, a Disabling Conditions Index was computed whereby the sum of all acceptance ratings on MP and MT inferences with many disabling conditions was subtracted from the sum of all acceptance ratings on MP and MT inferences with few disabling conditions. Hence, the higher the score, the greater the difference between acceptance ratings on the valid inferences (MP and MT) with many and few disabling conditions.
A multiple regression analysis was carried out with the Disabling Conditions Index as a dependent variable, with all 64 participants' scores on the four schizotypy factors entered as the independent variables. Nineteen percent of the variance was accounted for by the four factors, R(64) = 0.43, F(4,59) = 3.39, p = 0.01. The regression coefficients, t values, and significance levels for the individual factors are presented in table 5. It can be seen that ImpNon was the only factor that was significantly associated with the Disabling Conditions Index. The negative relationship indicates that high scores on the ImpNon factor were associated with low scores on the Disabling Conditions Index. Low scores on the disabling conditions index suggest that there is little impact of many or few disabling conditions on judgments on MP and MT inferences.
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Schizotypy and Alternatives. To examine the effects of schizotypy on alternative causes an Alternative Causes Index was computed whereby the sum of all acceptance ratings on AC and DA inferences with many alternative causes was subtracted from the sum of all acceptance ratings on AC and DA inferences with few alternative causes. Hence, the higher the score the greater the difference between acceptance ratings on inferences with many and few alternative causes. A multiple regression analysis was carried out with the Alternative Causes Index as a dependent variable and with the four factors of the O–LIFE as independent variables. Only 9 percent of the variance was explained by the schizotypy factors, which was not significant, R(64) = 0.30, F(4,59) = 1.48, p = 0.22. The regression coefficients, t values, and significance levels for the individual factors are presented in table 6. Examination of the regression coefficients revealed ImpNon to be the only significant factor in a negative direction. As with the Disabling Conditions Index, the negative relationship indicates that high scorers on the ImpNon factor were less influenced by the number of alternative causes on their judgments of AC and DA inferences. The fact that the coefficient and t value for ImpNon are lower for the Alternative Causes Index indicates that this effect of ignoring the number of counterexamples is not as strong for alternative causes as it is for disabling conditions.
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Schizotypy and Logic. A Logic Index was computed. The sum of the mean acceptance ratings for DA and AC inferences (the invalid inferences) was subtracted from the sum of the mean acceptance ratings for MP and MT arguments (the valid inferences) for each participant. Hence, the higher the score, the more logic-like the response pattern. A multiple regression analysis was carried out with the Logic Index as a dependent variable and the four factors of the O–LIFE as independent variables. Twenty-one percent of the variance was explained by the schizotypy factors, demonstrating a significant relationship, R(64) = 0.46, F(4,59) = 3.97, p = 0.007. The regression coefficients, t values, and significance levels for the individual factors are presented in table 7. This analysis confirms that high scores on the ImpNon factor were associated with a logic-like response pattern, while similar results were associated with low scores on the UnExp factor.
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Discussion |
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We successfully replicated the Cummins (1995)
study, which demonstrated that both alternative causes and disabling conditions affect people's reasoning judgments. Replicating her results is important because it means that any differences found can be attributed to schizotypy. Multiple regression analyses revealed that ImpNon was the only schizotypy factor consistently associated with reasoning performance. ImpNon was negatively associated with both the Disabling Conditions Index and the Alternative Causes Index. This result demonstrates that high scores on ImpNon were associated with ignoring counterexamples. The effect was strongest for disabling conditions, which suggests that high ImpNon scorers ignore the number of disabling conditions to a greater extent than they ignore the number of alternative causes. ImpNon also had a positive relationship with the Logic Index, indicating that high scorers exhibited more logic-like performance. Looking at these three results together leads to the conclusion that ignoring the influence of counterexamples, particularly disabling conditions (which influence MP and MT inferences), leads to logic-like performance on this task. Interestingly, examination of reaction time data revealed no significant effects. This finding confirmed that these effects were not due to high ImpNon scorers' being impulsive.
UnExp was also found to be negatively correlated with the Logic Index; low scorers exhibited more logic-like responses. However, as shown in table 1, UnExp (as well as IntAn) had significantly lower scores in this sample than in the normative data. It is possible that the association between UnExp scores and the Logic Index is a function of the low UnExp scores. Repeating this study with a sample who had scores more similar to the normative data would be the only way to clarify this issue.
No effects were observed with the remaining schizotypy factors (with the exception of UnExp and the Logic Index, discussed above). There are three possible explanations for this. First, the differences in schizotypy scores between this sample and the normative data may have confounded the results (as discussed above). Second, the remaining three factors, UnExp, IntAn, and CogDis, represent particular symptom dimensions in schizophrenia, whereas ImpNon is a general psychosis-proneness factor more allied with affective psychoses. If deficits in reasoning are associated with delusion formation and maintenance, as we have suggested, then factors associated with thought disorder (CogDis) and negative symptoms (represented by IntAn) may not affect this task. The only factor for which we might expect to see an effect is UnExp, which, as already mentioned, may be confounded by the low scores in this sample. The third explanation is that scores on schizotypy factors differ by gender. Males tend to have higher scores on ImpNon and IntAn, while females tend to have higher scores on UnExp and CogDis (Mason et al. 1995). In this study, there were no significant differences between gender for each schizotypy factor. This in itself may be a problem to the extent that disproportionate gender distribution may have a detrimental influence on the overall mean scores for each of the schizotypy factors. Furthermore, males and females perform differently on this task, irrespective of schizotypy scores. However, without an equal gender distribution, this hypothesis could not be investigated.
We now turn to the explanation of the findings for ImpNon. It has been suggested that a data-gathering bias may contribute to the formation and maintenance of delusions (Huq et al. 1988; Garety 1991; Garety and Hemsley 1994; Garety and Freeman 1999) because such a bias would lead to inaccurate judgments (Mujica-Parodi et al. 2001). Our results showed that high scorers on the ImpNon factor exhibited a data-gathering bias, by their failure to take account of implicit counterexamples in reaching a decision. Thus, high ImpNon scorers are jumping to conclusions on the basis of incomplete information, which Garety and colleagues argue may contribute to the etiology of delusions. ImpNon is thought to be related to some of the affective psychoses (Mason et al. 1997). The observation that delusions also occur in affective illnesses (e.g., schizoaffective disorder, delusional depression, psychotic affective illness) provides a direct link between the data-gathering bias seen in high scorers on ImpNon and delusion formation and maintenance (for not only affective psychoses but also schizophrenia).
The evidence from this study draws together some of the major issues outlined early in this article. The long-held view that affective psychoses and schizophrenia exist as separate entities has recently been challenged by those arguing that these illnesses may exist on a psychosis continuum (e.g., Crow 1986, 1991; Kendell 1991; Taylor 1992; Van Os et al. 2000; Johns and Van Os 2001). We further reviewed the arguments for three- and four-factor models of schizotypy, whereby proponents of three-factor models uphold schizotypy as a measure of liability to risk of schizophrenia. By contrast, supporters of four-factor models subscribe to the notion of a psychosis-proneness continuum. Our evidence, where effects were found on only the ImpNon factor (thought to measure more affective psychoses), together with the fact that delusions also occur in affective illnesses, supports the notion of a psychosis continuum and that four-factor models of schizotypy are more appropriately thought of as psychosis-proneness rather than proneness to schizophrenia.
While we support a psychosis continuum approach, our results do affect research into schizophrenia. The results reported here from a normal population using schizotypy measures are relevant to two previously unrelated areas of research with schizophrenia patients. First, these results directly support the assumption of a jump to conclusions reasoning bias, which may be responsible for delusion formation and maintenance. It has been previously shown that schizophrenia patients and individuals high in delusional ideation exhibit a jump to conclusions style of reasoning (Huq et al. 1988; Garety and Hemsley 1994; John and Dodgson 1994; Dudley et al. 1997; Linney et al. 1998; Garety and Freeman 1999). We demonstrated that high scorers on the ImpNon factor used less information when making reasoning judgments, as they failed to take account of the counterexamples implicitly available in semantic memory. Second, the data indirectly support previous research showing that patients are prone to overinclusive thinking (Cameron 1939, 1945; Chapman and Chapman 1973; Chen et al. 1994). We suggested earlier that ignoring exceptions is consistent with overinclusive thinking; that is, patients have been shown to ignore exceptions to a general category rule such as "If it flies, then it is a bird" when they include an exemplar such as "airplane" in the category "bird." The results from this study demonstrated that high ImpNon scorers clearly ignored exceptions (in this instance, in the form of disabling conditions), which lends indirect support to the argument that patients (or in the case of the present study, high ImpNon scorers) may be prone to overinclusive thinking.
Although we have not empirically tested semantic memory in this study, there is considerable evidence that human reasoning depends upon the ability to successfully use information stored in semantic memory (e.g., Cheng and Holyoak 1985, 1989; Byrne 1989; Cosmides 1989; Cummins et al. 1991; Manktelow and Over 1991, 1995; Gigerenzer and Hug 1992; Oaksford and Chater 1994, 1998; Thompson 1994; Cummins 1995; Stevenson and Over 1995; Quinn and Markovits 1998). Garety and Freeman (1999) suggest that the jump to conclusions style of reasoning seen in deluded patients is not a function of a memory deficit. However, the results from this study suggest otherwise, because the counterexamples that influenced reasoning judgments depended on information held in semantic memory. The ability to reason about the world and successfully draw inferences depends very much on the knowledge represented in semantic memory (Oaksford and Chater 1998). Reasoning and semantic memory are interdependent. High scorers on the ImpNon factor were not influenced by the number of counterexamples characterizing a cause-effect relationship. They failed to carry out the general reasoning strategy of searching for counterexamples that Cummins (1995) suggested people must do to reason accurately. We suggest that an inability to use information stored in semantic memory contributes to flawed reasoning judgments. Moreover, we believe that a combination of impairments in reasoning and semantic memory results in erroneous inferences that may lead to delusion formation and maintenance. However, further empirical studies of semantic memory carried out simultaneously with reasoning tasks are necessary to fully examine the interdependent nature of reasoning and semantic memory.
As discussed earlier, theoretical accounts of human reasoning have traditionally employed logic as a normative framework to explain how people should reason. There is already considerable evidence to suggest that this may not be the appropriate norm to adopt (e.g., Manktelow and Over 1991, 1995; Oaksford and Chater 1994, 1998; Stanovich 1999), and the data presented here support this. It is clear from this study as well as others (e.g., Byrne 1989; Cummins et al. 1991; Cummins 1995; Stevenson and Over 1995) that other information does influence people's reasoning strategies. Failure to take account of all possible information has, as we predicted, led high schizotypes to exhibit logic-like performance on the conditional inference task. If logic is the framework by which we assess "normal" reasoning performance and an atypical trait correlates with the normative response, then we have provided compelling evidence that the wrong norm has been adopted (see also Oaksford and Sellen 2000; Stanovich and West 2000).
To conclude, by employing schizotypy measures we have presented data that support the notion of a jump to conclusions style of reasoning, mediated, we argue, via a process of overinclusive thinking. This evidence was demonstrated in high scorers on the ImpNon factor and is consistent with research carried out with schizophrenia patients. Furthermore, it is argued that the reasoning bias observed in the high ImpNon scorers may be a result of either impaired storage, or poor access to and use of implicit information stored in semantic memory. Efficient storage and access strategies have been shown to be necessary for accurate human reasoning, and we argue that deficits in one or both of these systems may be a major contributor to delusion formation. The fact that reasoning biases were observed on ImpNon, which is thought to be a general psychosis factor, suggests that it may reflect an underlying disposition that contributes to delusion formation and maintenance. We acknowledge that the lower scores observed on the UnExp factor (compared to normative data) may be problematic, and clearly further research is needed with a sample whose UnExp profile is more closely aligned to the normative data. This would enable us to establish whether the findings are specific to a general psychosis-proneness factor (ImpNon) or whether they could be extrapolated more generally to other dimensions of schizotypy (e.g., UnExp).
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Appendix. Conditional statements on which inferences were based |
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TopAbstractConditional Inference Task and...Reasoning and Semantic MemoryRole of Reasoning Deficits...Schizophrenia, Schizotypy, and...MethodsResultsDiscussionAppendix. Conditional statements...References |
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Note.—Taken from Cummins (1995).
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Acknowledgments |
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This research was supported by a research studentship grant (No. R00429834674) from the Economic and Social Research Council, Postgraduate Training Division, Swindon, U.K. This grant was awarded to the first author to undertake study for a Ph.D. The authors would like to acknowledge the kind assistance of Denise Cummins in the selection of experimental materials and to thank Professor Gordon Claridge for his helpful comments on this article.
The data presented here have previously been presented at the Annual Conference of the British Psychological Society (April 2000), the biannual meeting of the Experimental Psychology Society (July 2000), and the Thinking 2000 Conference (August 2000).
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