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A cluster analytic study of schizotypal trait dimensions
~
Person. individ. Diff Vol. 23, No. 5, pp. 877-883, 1997 ~ 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain
Pergamon
PII: 80191-8869(97)00086-X
A CLUSTER
ANALYTIC STUDY OF SCHIZOTYPAL DIMENSIONS
o191-8869/97$17.oo+0.o0
TRAIT
C. M. Loughland* and L. M.Williams Department of Psychology, University of New England, Armidale, New South Wales 2351, Australia
(Received 15 July 1996; received.for publication 7 May 1997) Summary--Several factor analytic studies reveal the existence of three distinct dimensions of schizotypy that appear to reflect the disposition to distinct schizophrenic syndromes (e.g. Bentall, Claridge & Slade, British Journal of Clinical Psychology, 28, 363-375, 1989; Claridge, McCreery, Mason, Bentall, Boyle, Slade & Popplewell, British Journal of Clinical Psychology, 35, 103-115, 1996; Muntaner, Garcia-Sevilla, Fernandez & Torrubia, Personality and Individual Differences, 9, 257-268, 1988). Claridge et al. referred to the dimensions as 'Aberrant Perceptions and Beliefs', 'Cognitive Disorganisation' and 'Introvertive Anhedonia'. The present study sought to establish the ecological validity of the three dimensions by cluster analysing individual responses on the Oxford-Liverpool Inventory of Feelings and Experiences (OLIFE; Mason, Claridge & Jackson, Personality and Individual Differences, 18, 7-13, 1995). An agglomerative hierarchical cluster analysis of OLIFE subscale scores for 69 undergraduate psychology students revealed four separate clusters. Based on the defining feature(s) in each cluster, they were referred to as the Unusual Experiences, Cognitive Disorganisation, Introvertive Anhedonia, and Low Schizotypy subgroups. As indicated by these descriptive labels, three of the subgroups aligned broadly with previously defined dimensions of schizotypy, while the fourth subgroup reflected the overall absence of schizotypal traits. Thus, the results indicate that the three-dimensional structure of schizotypy suggested by Claridge et al. (1996) is reflected in naturally occurring subgroups of non-psychotic individuals. © 1997 Elsevier Science Ltd
INTRODUCTION Evidence from genetic and cognitive research into the borderline states of psychosis (e.g. Schizotypal Personality Disorder and Borderline Personality Disorder) points to a continuity of behaviour between those individuals diagnosed with a psychotic disorder, and non-psychotic individuals. In light of this evidence, Claridge (1985) developed a continuum model of schizophrenic aetiology. Claridge proposed that the latent vulnerability to schizophrenia manifests in the form of a continuum of cognitive and personality traits that may exist in the absence of manifest illness. Collectively, these traits have been termed 'schizotypy' and refer to an individual's proneness to psychosis, and in particular, to schizophrenia. Schizotypy is regarded as multidimensional, reflecting not only the heterogeneity found in schizophrenia, but also the likelihood that it has a multigenic basis. Although schizotypal traits constitute part of the normal variation found across different human characteristics, when present to a high degree, they may impair what is otherwise healthy cognitive, emotional, and social functioning. As such, particular configurations of schizotypal traits may provide a marker for the identification of non-psychotic individuals who are prone to psychosis (Chapman, Chapman & Raulin, 1976). Using factor analysis, psychometric research has directly examined the multidimensional structure of schizotypal traits as measured by the range of existing scales. Several factor analytic studies have produced two dimensions of schizotypal traits, largely reflecting the division between 'positive' and 'negative' traits (e.g. Kelley & Coursey, 1992; Raine & Allbutt, 1989). In these studies the 'positive' dimension was defined by high loadings on measures of ideational and perceptual disturbance, such as the Magical Ideation and Perceptual Aberration Scales (Chapman, Chapman & Raulin, 1978; Eckblad & Chapman, 1983), whereas the 'negative' dimension was characterised primarily by contributions from the Social and Physical anhedonia scales (Chapman et aI., 1976). Nevertheless, at least three dimensions have emerged from a number of other factor analytic studies (Bentall, Claridge & Slade, 1989; Claridge, McCreery, Mason, Bentall, Boyle, Slade &
*To whom all correspondence should be addressed. 877
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Popplewell, 1996; Kendler & Hewitt, 1992; Muntaner, Garcia-Sevilla, Fernandez & Torrubia, 1988). In each of these studies the 'positive' and 'negative' dimensions described above have emerged consistently, attesting to their stability across samples. An additional dimension, defined primarily by cognitive and attentional difficulties, has also been revealed. Given the nature of the items tapped by the schizotypy scales, the dimension has been referred to as one of cognitive disorganisation (Bentall et al., 1989; Claridge et al., 1996). Claridge et al. (1996) used principal components analysis with varimax rotation to factor analyse the responses of 1095 participants on 18 schizotypy scales. Given the specific loadings on the 'positive' and 'negative' dimensions, they were labelled 'Aberrant Perceptions and Beliefs' and 'Introvertive Anhedonia', respectively. The factor defined by attentional and social problems (representing the second factor in Claridge et al.'s solution) was referred to as one of 'Cognitive Disorganisation'. These three dimensions were regarded by Claridge et al. as representative of the three main facets of schizotypy. Moreover, it was suggested that the Aberrant Perceptions and Beliefs, Cognitive Disorganisation and Introvertive Anhedonia dimensions are paralleled in previously identified syndromes of schizophrenia, such as Liddle's (Liddle, 1987) Reality Distortion, Disorganisation, and Psychomotor Poverty syndromes of schizophrenia, respectively, That is, the configuration of positive and negative symptoms in each schizophrenic syndrome tends to be paralleled, albeit in milder form, in the schizotypal dimensions. The fourth factor revealed by Claridge et al., namely the 'Asocial Behaviour' dimension, was instead thought to be more closely aligned with affective psychosis than with schizophrenia p e r se. On the basis of Claridge et al.'s (1996) factor analytic solution, Mason, Claridge and Jackson (1995) developed a comprehensive measure of schizotypal trait dimensions, namely, OLIFE. Four OLIFE subscales, intended to index distinct schizotypy dimensions, were formed by selecting items that had loadings of 0.28 or greater on Claridge et al.'s (1996) four factors. To reflect their item content, the subscales were referred to as the 'Unusual Experiences' (aberrant perceptions and beliefs), 'Cognitive Disorganisation', 'Introvertive Anhedonia' and 'Impulsive Nonconformity' (asocial behaviour) subscales (see Materials section for description of subscales). The object of our study was to establish the ecological validity of Claridge et al.'s (1996) three primary dimensions of schizotypy, as indexed by the Unusual Experiences, Introvertive Anhedonia, and Cognitive Disorganisation subscales of the OLIFE. The term 'ecological validity' is commonly used to refer to the extent to which measurement data represent the actual construct of interest as it occurs in naturalistic settings (e.g. Barkley, 1991). Our use of the term reflects a parallel consideration, namely, the extent to which the three dimensions of schizotypy are manifested in naturally occurring groups of individuals. To this end, we subjected individuals' scores on the OLIFE subscales to cluster analysis. As cluster analysis is not often used in psychometric research a brief outline of its relevance to our study is warranted. Factor analysis provides information about the dimensional structure of schizotypy in terms of the interrelationships between measures of schizotypal traits. But, it does not tell us how individuals might be grouped according to their responses on these measures. Cluster analysis, on the other hand, can be used to identify distinct groups of individuals, defined by different configurations of schizotypal traits. The value of this technique is that the groups (commonly referred to as 'types') are identified on the basis of natural groupings in the data themselves (in our case, schizotypy subscale scores). Cluster analysis can therefore be thought of as a way of producing a 'numerical taxonomy' (or typology) of people with regard to their profile of schizotypal traits (Hair, Anderson, Tatham & Black, 1992). Ecological validity can be evaluated in terms of the extent to which the cluster typology mirrors the factor structure of schizotypy. Several studies have used cluster analysis to develop a typology of schizophrenic symptoms (e.g. Farmer, McGuffin & Spitznagel, 1983; Gur, Mozley, Resnick, Levick, Erwin, Saykin & Gur, 1991; Morrison, Bellack, Wixted & Mueser, 1990; Sauer, Geider, Binkert, Reitz & Schroder, 1991; Van der Does, Linszen, Dingemans, Nugter & Scholte, 1993; Williams, 1996). However, Williams' (1994) study is the only attempt to date to establish a typology of schizotypal traits. Williams analysed measures of single schizotypal traits, selected to tap the three schizotypy dimensions revealed by Bentall et al. (1989). Two measures, the Magical Ideation and Physical Anhedonia scales (Eckblad & Chapman, 1983; Chapman et al., 1976), were chosen because they loaded consistently on the unusual experiences and introvertive anhedonia dimensions of schizotypy, respectively. The STA
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and Revised Social Anhedonia Scale (Eckblad, Chapman, Chapman & Mishlove, 1982) were also included. The STA was chosen because it defines the more equivocal cognitive disorganisation dimension of schizotypy as well as the unusual experiences dimension (Bentall et al., 1989). Williams' argument for including the Revised Social Anhedonia scale was that it would determine whether people's experience of social withdrawal is associated with their grouping in terms of an introvertive anhedonia dimension or, instead, if social withdrawal might contribute to a separate cluster (along with the STA) that reflects the cognitive disorganisation dimension. The possibility that social anhedonia may be associated with cognitive disorganisation when manifested in individuals arose from Kendler and Hewitt's (Kendler & Hewitt, 1992) finding that social anhedonia loaded with the cognitive aspects of the STA, whereas in other studies it loaded with the Physical Anhedonia scale (e.g. Bentall et al., 1989; Claridge et al., 1996). Cluster analysis of 70 participants' responses on the four scales produced four distinct clusters (Williams, 1994). Cluster 1 was defined by a low level of schizotypy generally. By contrast the other three clusters were delineated by above average scores on at least one schizotypy trait. Cluster 2 comprised individuals with high scores on the Magical Ideation and STA scales, and was thus characterised by unusual ideational and perceptual experiences. Cluster 3 consisted of individuals with high scores on the Social Anhedonia scale as well as the STA, while cluster 4, on the other hand, was defined by individuals with high scores on the Physical Anhedonia scale only. According to Williams, these clusters were therefore characterised by cognitive disorganisation and physical anhedonia, respectively. Williams noted further that the three 'high schizotypy' clusters showed a broad correspondence with Bentall et al.'s (1989) schizotypy dimensions, indicating that individuals may indeed be grouped along these dimensions, with only minor variations. The main departure from Bentall et al.'s dimensional structure of schizotypy is that people's experience of social anhedonia appears to be related more to aspects of cognitive disorganisation than to physical anhedonia. This finding was nonetheless consistent with that of Kendler & Hewitt (1992), mentioned previously. Despite the fact that Williams' (1994) results provide some support for the ecological validity of the three schizotypy dimensions, her study is somewhat limited by the use of single trait measures, rather than a more comprehensive measure of each of the dimensions. Our study has overcome this limitation in the use of the three OLIFE subscales. It was nonetheless expected that the typology produced by cluster analysis of OLIFE subscales would mirror the factor structure of schizotypy in a way analogous to Williams' cluster analysis of individuals trait scales. That is, on the basis of Williams' findings, it was expected that four clusters would emerge; three reflecting the dimensions of unusual experiences (or aberrant perceptions and beliefs), cognitive disorganisation, and introvertive anhedonia, and a fourth representing a relative absence of schizotypal traits. METHOD Participants
A total of 69 undergraduate psychology students from the University of New England, Armidale, took part in the study on a voluntary basis. The exclusion criterion for students was a previous history of psychosis, indicated by either a relevant diagnosis, period(s) of hospitalisation, or drug therapy. The sample comprised 47 females and 22 males; the mean age was 24.7 years (SD = 8.7; range 17-53 years). Materials
Three subscales from Mason et al.'s (1995) OLIFE were used to index distinct facets of schizotypy (see Mason et aL for a description of OLIFE's development). The Unusual Experience subscale comprises 30 items that tap aberrant perceptual and cognitive experiences, and odd beliefs. These features of schizotypal personality are thought to reflect the positive symptoms of schizophrenia (Bentall et al., 1989). The Cognitive Disorganisation subscale includes 24 items designed to measure cognitive and attentional difficulties, along with social anxiety and emotional sensitivity. These features of schizotypy appear to correspond to cognitively oriented positive symptoms of schizophrenia in particular (e.g. thought disorder). The Introvertive Anhedonia subscale comprises 27
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items tapping lack of enjoyment in social contact and physical sources of pleasure. These schizotypal features feasibly relate to negative schizophrenic symptomatology, particularly 'deficit' negative symptoms such as anhedonia and avolition (Carpenter, Heinrichs & Wagman, 1988). The remaining Impulsive Nonconformity subscale of the OLIFE was not used in our study because it concerns characteristics that are not central to schizotypy p e r se. Psychometric evaluation of the OLIFE has shown it to have good test-retest reliability (coefficient alpha = 0.80), as well as acceptable internal reliability (coefficient alpha-0.70). RESULTS Means (and standard deviations) for each subscale were 10.68 (5.89) for Unusual Experiences, 12.01 (5.43) for Cognitive Disorganisation, and 5.33 (3.53) for Introvertive Anhedonia. An agglomerative hierarchical cluster analysis was conducted to explore the existence of a typology of participants based on their OLIFE subscale scores. Since the range of possible scores varied across subscales, standardised scores were employed in this analysis. Ward's minimum variance method of cluster analysis was employed with similarities between participant profiles defined by squared Euclidean distance. To clarify the number of clusters appropriate for analysis, a scree-type plot was employed. A plot of the successive differences between the last 12 cluster merges revealed the first large increment (first plotted point off the scree line) in aggregate Euclidean distance occurred at the merging of four clusters into three clusters, suggesting that the four cluster solution was the appropriate solution to interpret. Standardised means for the Unusual Experiences (UE), Cognitive Disorganisation (CD), and Introvertive Anhedonia (IA) subscales, and the amount of variance accounted for by the four clusters 0l 2) are shown in MANOVA indicated that there were significant differences (each at p < 0.0001) across clusters for Unusual Experiences (F~3,65)= 38.43), Cognitive Disorganisation (F~3, 65~= 14.99), and Introvertive Anhedonia (F(3 ' 65)= 49.39). These differences, along with the r/z values, attest to the internal validity of the cluster solution. Using the standardised scores from Fig. 1, the following descriptive interpretation of clusters was made. Cluster 1 comprised 19 individuals with generally below average scores for each subscale and thus a relatively low level of schizotypy overall. Cluster 2 comprised 27 individuals with a relatively high level of unusual experiences, an average level of cognitive disorganisation and a below average level of introvertive anhedonia. The 18 individuals in Cluster 3 were distinguished by a relatively high level of cognitive disorganisation along with an average level of both unusual experiences and introvertive anhedonia. The fourth and smallest cluster consisted of five individuals characterised by an extremely high level of introvertive anhedonia, a high level of cognitive disorganisation, and an average level of unusual experiences. Thus, on the basis of their dominant schizotypal feature, these clusters were referred to as the 'Unusual Experiences', 'Cognitive Disorganisation', 'Introvertive Anhedonia', and 'Low Schizotypy' subgroups, respectively. DISCUSSION Cluster analytic results revealed the existence of four distinct subgroups of individuals, characterised by different patterns of scores on the three indices of schizotypy. These findings not only provide evidence for the multidimensional nature of schizotypal traits, but also attest to the ecological validity of the factorial structure of schizotypy. The first subgroup consisted of individuals with relatively low scores on all three subscales. This cluster may therefore represent a general lack of vulnerability for schizophrenic symptomatology. In the second cluster, however, individuals exhibited predominantly high scores on the unusual experiences subscale of the OLIFE; this cluster was therefore referred to as the 'Unusual Experiences' subgroup. Given that the unusual experiences subscale measures primarily positive schizotypal features, such as magical thinking and perceptual distortions, this subgroup could be aligned with the reality distortion syndrome of positive schizophrenic symptoms (Liddle, 1987). For instance, magical thinking feasibly is a milder version of delusional thoughts and perceptual distortions, an attenuated version of hallucinations.
Schizotypal trait dimensions
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Cluster 1 (n=19)
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Cluster 2 (n=27)
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Cluster 3 (n=18)
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Cluster 4 (n=5)
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Fig. 1. Mean z-scorefor cluster on schizotypyscales.
In the third cluster individuals were distinguished by high scores on the cognitive disorganisation subscale. They also displayed moderately high scores on both the unusual experiences and introvertive anhedonia subscales of the OLIFE. On the basis of the predominant characteristics, the cluster was referred to as the 'Cognitive Disorganisation' subgroup. The configuration of cognitive disorganisation, in combination with more general positive traits and certain negative schizotypal features, suggests an alignment with the Disorganisation syndrome of schizophrenia (Liddle, 1987). The Unusual Experiences and Cognitive Disorganisation subgroups therefore share, to some extent, the ideational and perceptual aspects of schizotypy in a manner analogous to the overlap in hallucinations and delusions between the Reality Distortion and Disorganisation syndromes of schizophrenia (Williams, 1996). Cluster 4 was differentiated by individuals with very high scores on the introvertive anhedonia subscale; they also displayed moderately high scores on the cognitive disorganisation subscale of the OLIFE. This cluster was therefore termed the 'Introvertive Anhedonia' subgroup. Given that this subgroup is characterised by purely negative schizotypal traits, it appears to correspond with the psychomotor poverty syndrome of negative schizophrenic symptoms (Liddle, 1987). However, given the small ( n - 5 ) number of participants in the Introvertive Anhedonia subgroup, such an interpretation must be regarded as somewhat tentative. We nevertheless gain some confidence from the fact that participants' high scores on the Introvertive Anhedonia subscale showed only moderate variation (SD = 0.56). Although the Unusual Experiences, Cognitive Disorganisation and lntrovertive Anhedonia subgroups were defined predominantly by elevated scores on distinct schizotypal traits, it is nonetheless worth noting the degree of overlap between these subgroups in non-defining traits. The contribution of ideational and perceptual disturbances (which define the Unusual Experiences subgroup) to the Cognitive Disorganisation subgroup was mentioned above in the context of parallelism between these schizotypal subgroups and the schizophrenic syndromes of Reality Distortion and Disorganisation, respectively. In the schizophrenic Disorganisation syndrome, the presence of thought
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C.M. Loughland and L. M. Williams
disorder in addition to hallucinations and delusions is indicative of a particularly severe form of the disorder (Bilder, Mukherjee, Rieder & Pandurangi, 1995). A less straightforward finding is the contribution of cognitive disorganisation to the Introvertive Anhedonia subgroup, and of introvertive anhedonia to the Cognitive Disorganisation subgroup. Such overlap between these subgroups might be explicable in terms of the existence of social anxiety in each cluster; perhaps different forms of social anxiety. Kretschmer (1925, cited in Mishlove & Chapman, 1985) differentiated characteristics of social anxiety in terms of hyperaesthetic and anaesthetic features. According to Kretschmer (1925, cited in Mishlove & Chapman, p. 385), hyperaesthetic anxiety results from a "hypersensitivity to potential rejection, humiliation and shame". Anaesthetic anxiety, on the other hand, refers to social withdrawal that results from a personal indifference to people. Given this distinction, it is possible that the Cognitive Disorganisation subgroup is delineated by a non-clinical manifestation of the hyperaesthetic form of social anxiety, as reflected in the fundamentally positive characteristics of this subgroup, namely, the personality characteristics of suspiciousness and paranoia. This particular subgroup may manifest social anxiety as a secondary consequence of their mild positive symptomatology and bizarre ideas. That is, they may deliberately withdraw socially as a way of coping with or minimising their suspicious and paranoid thoughts and beliefs. The Introvertive Anhedonia subgroup, on the other hand, might be more precisely delineated by a non-clinical anaesthetic form of social anxiety, as reflected in the personality characteristics of schizoid withdrawal and anhedonia in this subgroup. That is, this subgroups' social anxiety might represent a primary schizotypal feature that is part of the anhedonic personality type. Thus, social anxiety as a schizotypal feature may contribute to both the Cognitive Disorganisation and Introvertive Anhedonia subgroups, but in qualitatively different ways. Our findings are not only consistent with previous cluster analytic results (Williams, 1994), but also correspond broadly with those from factor analyses. In terms of cluster analytic research, the results support Williams' finding that individuals may be grouped into four schizotypal trait subgroups. The Low Schizotypy, Unusual Experiences, Introvertive Anhedonia, and Cognitive Disorganisation subgroups of the present study are directly comparable to Williams' Clusters 1 to 4, respectively. As in Williams' study, the configuration of schizotypal features in each of our subgroups suggested a phenomenological correspondence with Liddle's (1987) schizophrenic syndromes of reality distortion, disorganisation, and psychomotor poverty. Our findings therefore, not only confirm the multidimensional nature of schizotypy in terms of how it exists within individuals, but also add weight to the notion that Liddle's schizophrenic syndromes represent p r i m a r y syndromes that have some basis in normal personality, rather than reflect secondary consequences of the schizophrenic process. In relation to factor analytic findings, the results attest to the ecological validity of the tridimensional structure of schizotypy proposed by Claridge et al. (1996). That is, our cluster typology indicates that Claridge et al.'s factors of Aberrant Perceptions and Beliefs (or Unusual Experiences), Cognitive Disorganisation and Introvertive Anhedonia, respectively, are broadly manifested within individuals. To investigate the association between schizotypal trait subgroups in the vulnerability to distinct syndromes of psychosis, there is a need to validate the present typology in a clinical sample. As a means of externally validating such an association, future studies might also determine whether corresponding schizotypal and schizophrenic subgroups perform similarly on cognitive tasks. The present authors are currently undertaking such a study. The value of establishing the validity of schizotypal subgroups as precursors of particular schizophrenic syndromes is that subsequent research into schizophrenic processes may be conducted confidently on non-clinical samples. In this way the confounding effects of drug therapy, institutionalisation and separation from the community are avoided.
REFERENCES Barkley,R. A. (1991). The ecologicalvalidityof laboratoryand analogueassessmentmethodsof ADHD symptoms.Journal of Abnormal Child Psychology, 19, 149 178.
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Bentall, R. P., Claridge, G. S. & Slade, P. D. (1989). The multidimensional nature of schizotypal traits: A factor analytic study with normal subjects. British Journal of Clinical Psychology, 28, 363-375. Bilder, R. M., Mukherjee, S., Rieder, R. O. & Pandurangi, A. K. (1995). Symptomatic and Neuropsychological correlates of defect states. Schizophrenia Bulletin, 11,409-4 19. Carpenter, W. T., Heinrichs, D. W. & Wagman, A. M. I. (1988). Deficit and nondeficit forms of schizophrenia: The concept. American Journal of Psychiatry, 145, 578-583. Chapman, L. J., Chapman, J. P. & Raulin, M. L. (1976). Scales for physical and social anhedonia. Journal of Abnormal Psychology, 85, 374-382. Chapman, L. J., Chapman, J. P. & Raulin, M. L. (1978). Body-image aberration in schizophrenia. Journal of Abnormal Psychology, 87, 399-407. Claridge, G. (1985). Origins of mental illness. Oxford: Blackwell. Claridge, G., McCreery, C., Mason, O., Bentall, R., Boyle, G., Slade, P. & Popplewell, D. (1996). The factor structure of 'schizotypal' traits: A large replication study. British Journal of Clinical Psychology, 35, 103-115. Eckblad, M. & Chapman, L. J. (1983). Magical ideation as a indicator of schizotypy. Journal of Consulting and Clinical Psychology, 51, 215-225. Eckblad, M. L., Chapman, L. J., Chapman, J. P. & Mishlove, M. (1982). The revised social anhedonia scale. Unpublished test. University of Wisconsin-Madison. Farmer, A. E., McGuffin, P. & Spitznagel, E. L. (1983). Heterogeneity in schizophrenia. A cluster-analytic approach. Psychiatry Research, 8, 1 12. Gur, R. E., Mozley, P. D., Resnick, S. M., Levick, S., Erwin, R., Saykin, A. J. & Gur, R. C. (1991). Relations among clinical scales in schizophrenia. American Journal of Psychiatry, 148, 472-478. Hair, J. F., Anderson, R. E., Tatham, R. L. & Black, W. C. (1992). Multivariate data analysis with readings (3rd ed.). New York: MacMillan. Kelley, M. P. & Coursey, R. D. (1992). Factor structure of schizotypy scales. Personali o, and Individual Differences, 13, 723 731. Kendler, K. S. & Hewitt, J. (1992). The structure of self-report schizotypy in twins. Journal of Personality Disorders, 6, 1 17. Liddle, P. F. (1987). Schizophrenic syndromes, cognitive performance, and neurological dysfunction. Psychological Medicine, 17, 49 57. Mason, O., Claridge, G. & Jackson, M. (1995). New scales for the assessment of schizotypy. Personality and Individual Differences, 18, 7 13. Mishlove, M. & Chapman, L. J. (1985). Social anhedonia in the prediction of psychosis proneness. Journal of Abnormal Psycholoyy, 94, 384-396. Morrison, R. L., Bellack, A. S., Wixted, J. T. & Mueser, K. T. (1990). Positive and negative symptoms in schizophrenia. A cluster-analytic approach. Journal of Nervous and Mental Disease, 178, 377 384. Muntaner, C., Garcia-Sevilla, L., Fernandez, A. & Torrubia, R. (1988). Personality dimensions, schizotypal and borderline personality traits and psychosis proneness. Personality and Individual Differences, 9, 257-268. Raine, A. & Allbutt, J. (1989). Factors of schizoid personality. British Journal of Clinical Psychology, 28, 31-40. Sauer, H., Geider, F. J., Binkert, M., Reitz, C. & Schroder, J. (1991). Is chronic schizophrenia heterogenous? Bioloyical Psychiatry, 29, 661S. Van der Does, A. J. W., Linszen, D. H., Dingemans, P. M., Nugter, M. A. & Scholte, W. F. (1993). A dimensional and categorical approach to the symptomatology of recent-onset schizophrenia. Journal of Nervous and Mental Disease, 181, 744-749. Williams, L. M. (1994). The multidimensional nature of schizotypy traits: A cluster analytic study. Personality and Individual Differences, 16, 103-112. Williams, L. M. (1996). Cognitive inhibition and schizophrenic symptoms subgroups. Schizophrenia Bulletin, 22, 139 151.
Person. individ. Diff Vol. 23, No. 5, pp. 877-883, 1997 ~ 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain
Pergamon
PII: 80191-8869(97)00086-X
A CLUSTER
ANALYTIC STUDY OF SCHIZOTYPAL DIMENSIONS
o191-8869/97$17.oo+0.o0
TRAIT
C. M. Loughland* and L. M.Williams Department of Psychology, University of New England, Armidale, New South Wales 2351, Australia
(Received 15 July 1996; received.for publication 7 May 1997) Summary--Several factor analytic studies reveal the existence of three distinct dimensions of schizotypy that appear to reflect the disposition to distinct schizophrenic syndromes (e.g. Bentall, Claridge & Slade, British Journal of Clinical Psychology, 28, 363-375, 1989; Claridge, McCreery, Mason, Bentall, Boyle, Slade & Popplewell, British Journal of Clinical Psychology, 35, 103-115, 1996; Muntaner, Garcia-Sevilla, Fernandez & Torrubia, Personality and Individual Differences, 9, 257-268, 1988). Claridge et al. referred to the dimensions as 'Aberrant Perceptions and Beliefs', 'Cognitive Disorganisation' and 'Introvertive Anhedonia'. The present study sought to establish the ecological validity of the three dimensions by cluster analysing individual responses on the Oxford-Liverpool Inventory of Feelings and Experiences (OLIFE; Mason, Claridge & Jackson, Personality and Individual Differences, 18, 7-13, 1995). An agglomerative hierarchical cluster analysis of OLIFE subscale scores for 69 undergraduate psychology students revealed four separate clusters. Based on the defining feature(s) in each cluster, they were referred to as the Unusual Experiences, Cognitive Disorganisation, Introvertive Anhedonia, and Low Schizotypy subgroups. As indicated by these descriptive labels, three of the subgroups aligned broadly with previously defined dimensions of schizotypy, while the fourth subgroup reflected the overall absence of schizotypal traits. Thus, the results indicate that the three-dimensional structure of schizotypy suggested by Claridge et al. (1996) is reflected in naturally occurring subgroups of non-psychotic individuals. © 1997 Elsevier Science Ltd
INTRODUCTION Evidence from genetic and cognitive research into the borderline states of psychosis (e.g. Schizotypal Personality Disorder and Borderline Personality Disorder) points to a continuity of behaviour between those individuals diagnosed with a psychotic disorder, and non-psychotic individuals. In light of this evidence, Claridge (1985) developed a continuum model of schizophrenic aetiology. Claridge proposed that the latent vulnerability to schizophrenia manifests in the form of a continuum of cognitive and personality traits that may exist in the absence of manifest illness. Collectively, these traits have been termed 'schizotypy' and refer to an individual's proneness to psychosis, and in particular, to schizophrenia. Schizotypy is regarded as multidimensional, reflecting not only the heterogeneity found in schizophrenia, but also the likelihood that it has a multigenic basis. Although schizotypal traits constitute part of the normal variation found across different human characteristics, when present to a high degree, they may impair what is otherwise healthy cognitive, emotional, and social functioning. As such, particular configurations of schizotypal traits may provide a marker for the identification of non-psychotic individuals who are prone to psychosis (Chapman, Chapman & Raulin, 1976). Using factor analysis, psychometric research has directly examined the multidimensional structure of schizotypal traits as measured by the range of existing scales. Several factor analytic studies have produced two dimensions of schizotypal traits, largely reflecting the division between 'positive' and 'negative' traits (e.g. Kelley & Coursey, 1992; Raine & Allbutt, 1989). In these studies the 'positive' dimension was defined by high loadings on measures of ideational and perceptual disturbance, such as the Magical Ideation and Perceptual Aberration Scales (Chapman, Chapman & Raulin, 1978; Eckblad & Chapman, 1983), whereas the 'negative' dimension was characterised primarily by contributions from the Social and Physical anhedonia scales (Chapman et aI., 1976). Nevertheless, at least three dimensions have emerged from a number of other factor analytic studies (Bentall, Claridge & Slade, 1989; Claridge, McCreery, Mason, Bentall, Boyle, Slade &
*To whom all correspondence should be addressed. 877
878
C.M. Loughland and L. M. Williams
Popplewell, 1996; Kendler & Hewitt, 1992; Muntaner, Garcia-Sevilla, Fernandez & Torrubia, 1988). In each of these studies the 'positive' and 'negative' dimensions described above have emerged consistently, attesting to their stability across samples. An additional dimension, defined primarily by cognitive and attentional difficulties, has also been revealed. Given the nature of the items tapped by the schizotypy scales, the dimension has been referred to as one of cognitive disorganisation (Bentall et al., 1989; Claridge et al., 1996). Claridge et al. (1996) used principal components analysis with varimax rotation to factor analyse the responses of 1095 participants on 18 schizotypy scales. Given the specific loadings on the 'positive' and 'negative' dimensions, they were labelled 'Aberrant Perceptions and Beliefs' and 'Introvertive Anhedonia', respectively. The factor defined by attentional and social problems (representing the second factor in Claridge et al.'s solution) was referred to as one of 'Cognitive Disorganisation'. These three dimensions were regarded by Claridge et al. as representative of the three main facets of schizotypy. Moreover, it was suggested that the Aberrant Perceptions and Beliefs, Cognitive Disorganisation and Introvertive Anhedonia dimensions are paralleled in previously identified syndromes of schizophrenia, such as Liddle's (Liddle, 1987) Reality Distortion, Disorganisation, and Psychomotor Poverty syndromes of schizophrenia, respectively, That is, the configuration of positive and negative symptoms in each schizophrenic syndrome tends to be paralleled, albeit in milder form, in the schizotypal dimensions. The fourth factor revealed by Claridge et al., namely the 'Asocial Behaviour' dimension, was instead thought to be more closely aligned with affective psychosis than with schizophrenia p e r se. On the basis of Claridge et al.'s (1996) factor analytic solution, Mason, Claridge and Jackson (1995) developed a comprehensive measure of schizotypal trait dimensions, namely, OLIFE. Four OLIFE subscales, intended to index distinct schizotypy dimensions, were formed by selecting items that had loadings of 0.28 or greater on Claridge et al.'s (1996) four factors. To reflect their item content, the subscales were referred to as the 'Unusual Experiences' (aberrant perceptions and beliefs), 'Cognitive Disorganisation', 'Introvertive Anhedonia' and 'Impulsive Nonconformity' (asocial behaviour) subscales (see Materials section for description of subscales). The object of our study was to establish the ecological validity of Claridge et al.'s (1996) three primary dimensions of schizotypy, as indexed by the Unusual Experiences, Introvertive Anhedonia, and Cognitive Disorganisation subscales of the OLIFE. The term 'ecological validity' is commonly used to refer to the extent to which measurement data represent the actual construct of interest as it occurs in naturalistic settings (e.g. Barkley, 1991). Our use of the term reflects a parallel consideration, namely, the extent to which the three dimensions of schizotypy are manifested in naturally occurring groups of individuals. To this end, we subjected individuals' scores on the OLIFE subscales to cluster analysis. As cluster analysis is not often used in psychometric research a brief outline of its relevance to our study is warranted. Factor analysis provides information about the dimensional structure of schizotypy in terms of the interrelationships between measures of schizotypal traits. But, it does not tell us how individuals might be grouped according to their responses on these measures. Cluster analysis, on the other hand, can be used to identify distinct groups of individuals, defined by different configurations of schizotypal traits. The value of this technique is that the groups (commonly referred to as 'types') are identified on the basis of natural groupings in the data themselves (in our case, schizotypy subscale scores). Cluster analysis can therefore be thought of as a way of producing a 'numerical taxonomy' (or typology) of people with regard to their profile of schizotypal traits (Hair, Anderson, Tatham & Black, 1992). Ecological validity can be evaluated in terms of the extent to which the cluster typology mirrors the factor structure of schizotypy. Several studies have used cluster analysis to develop a typology of schizophrenic symptoms (e.g. Farmer, McGuffin & Spitznagel, 1983; Gur, Mozley, Resnick, Levick, Erwin, Saykin & Gur, 1991; Morrison, Bellack, Wixted & Mueser, 1990; Sauer, Geider, Binkert, Reitz & Schroder, 1991; Van der Does, Linszen, Dingemans, Nugter & Scholte, 1993; Williams, 1996). However, Williams' (1994) study is the only attempt to date to establish a typology of schizotypal traits. Williams analysed measures of single schizotypal traits, selected to tap the three schizotypy dimensions revealed by Bentall et al. (1989). Two measures, the Magical Ideation and Physical Anhedonia scales (Eckblad & Chapman, 1983; Chapman et al., 1976), were chosen because they loaded consistently on the unusual experiences and introvertive anhedonia dimensions of schizotypy, respectively. The STA
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and Revised Social Anhedonia Scale (Eckblad, Chapman, Chapman & Mishlove, 1982) were also included. The STA was chosen because it defines the more equivocal cognitive disorganisation dimension of schizotypy as well as the unusual experiences dimension (Bentall et al., 1989). Williams' argument for including the Revised Social Anhedonia scale was that it would determine whether people's experience of social withdrawal is associated with their grouping in terms of an introvertive anhedonia dimension or, instead, if social withdrawal might contribute to a separate cluster (along with the STA) that reflects the cognitive disorganisation dimension. The possibility that social anhedonia may be associated with cognitive disorganisation when manifested in individuals arose from Kendler and Hewitt's (Kendler & Hewitt, 1992) finding that social anhedonia loaded with the cognitive aspects of the STA, whereas in other studies it loaded with the Physical Anhedonia scale (e.g. Bentall et al., 1989; Claridge et al., 1996). Cluster analysis of 70 participants' responses on the four scales produced four distinct clusters (Williams, 1994). Cluster 1 was defined by a low level of schizotypy generally. By contrast the other three clusters were delineated by above average scores on at least one schizotypy trait. Cluster 2 comprised individuals with high scores on the Magical Ideation and STA scales, and was thus characterised by unusual ideational and perceptual experiences. Cluster 3 consisted of individuals with high scores on the Social Anhedonia scale as well as the STA, while cluster 4, on the other hand, was defined by individuals with high scores on the Physical Anhedonia scale only. According to Williams, these clusters were therefore characterised by cognitive disorganisation and physical anhedonia, respectively. Williams noted further that the three 'high schizotypy' clusters showed a broad correspondence with Bentall et al.'s (1989) schizotypy dimensions, indicating that individuals may indeed be grouped along these dimensions, with only minor variations. The main departure from Bentall et al.'s dimensional structure of schizotypy is that people's experience of social anhedonia appears to be related more to aspects of cognitive disorganisation than to physical anhedonia. This finding was nonetheless consistent with that of Kendler & Hewitt (1992), mentioned previously. Despite the fact that Williams' (1994) results provide some support for the ecological validity of the three schizotypy dimensions, her study is somewhat limited by the use of single trait measures, rather than a more comprehensive measure of each of the dimensions. Our study has overcome this limitation in the use of the three OLIFE subscales. It was nonetheless expected that the typology produced by cluster analysis of OLIFE subscales would mirror the factor structure of schizotypy in a way analogous to Williams' cluster analysis of individuals trait scales. That is, on the basis of Williams' findings, it was expected that four clusters would emerge; three reflecting the dimensions of unusual experiences (or aberrant perceptions and beliefs), cognitive disorganisation, and introvertive anhedonia, and a fourth representing a relative absence of schizotypal traits. METHOD Participants
A total of 69 undergraduate psychology students from the University of New England, Armidale, took part in the study on a voluntary basis. The exclusion criterion for students was a previous history of psychosis, indicated by either a relevant diagnosis, period(s) of hospitalisation, or drug therapy. The sample comprised 47 females and 22 males; the mean age was 24.7 years (SD = 8.7; range 17-53 years). Materials
Three subscales from Mason et al.'s (1995) OLIFE were used to index distinct facets of schizotypy (see Mason et aL for a description of OLIFE's development). The Unusual Experience subscale comprises 30 items that tap aberrant perceptual and cognitive experiences, and odd beliefs. These features of schizotypal personality are thought to reflect the positive symptoms of schizophrenia (Bentall et al., 1989). The Cognitive Disorganisation subscale includes 24 items designed to measure cognitive and attentional difficulties, along with social anxiety and emotional sensitivity. These features of schizotypy appear to correspond to cognitively oriented positive symptoms of schizophrenia in particular (e.g. thought disorder). The Introvertive Anhedonia subscale comprises 27
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items tapping lack of enjoyment in social contact and physical sources of pleasure. These schizotypal features feasibly relate to negative schizophrenic symptomatology, particularly 'deficit' negative symptoms such as anhedonia and avolition (Carpenter, Heinrichs & Wagman, 1988). The remaining Impulsive Nonconformity subscale of the OLIFE was not used in our study because it concerns characteristics that are not central to schizotypy p e r se. Psychometric evaluation of the OLIFE has shown it to have good test-retest reliability (coefficient alpha = 0.80), as well as acceptable internal reliability (coefficient alpha-0.70). RESULTS Means (and standard deviations) for each subscale were 10.68 (5.89) for Unusual Experiences, 12.01 (5.43) for Cognitive Disorganisation, and 5.33 (3.53) for Introvertive Anhedonia. An agglomerative hierarchical cluster analysis was conducted to explore the existence of a typology of participants based on their OLIFE subscale scores. Since the range of possible scores varied across subscales, standardised scores were employed in this analysis. Ward's minimum variance method of cluster analysis was employed with similarities between participant profiles defined by squared Euclidean distance. To clarify the number of clusters appropriate for analysis, a scree-type plot was employed. A plot of the successive differences between the last 12 cluster merges revealed the first large increment (first plotted point off the scree line) in aggregate Euclidean distance occurred at the merging of four clusters into three clusters, suggesting that the four cluster solution was the appropriate solution to interpret. Standardised means for the Unusual Experiences (UE), Cognitive Disorganisation (CD), and Introvertive Anhedonia (IA) subscales, and the amount of variance accounted for by the four clusters 0l 2) are shown in MANOVA indicated that there were significant differences (each at p < 0.0001) across clusters for Unusual Experiences (F~3,65)= 38.43), Cognitive Disorganisation (F~3, 65~= 14.99), and Introvertive Anhedonia (F(3 ' 65)= 49.39). These differences, along with the r/z values, attest to the internal validity of the cluster solution. Using the standardised scores from Fig. 1, the following descriptive interpretation of clusters was made. Cluster 1 comprised 19 individuals with generally below average scores for each subscale and thus a relatively low level of schizotypy overall. Cluster 2 comprised 27 individuals with a relatively high level of unusual experiences, an average level of cognitive disorganisation and a below average level of introvertive anhedonia. The 18 individuals in Cluster 3 were distinguished by a relatively high level of cognitive disorganisation along with an average level of both unusual experiences and introvertive anhedonia. The fourth and smallest cluster consisted of five individuals characterised by an extremely high level of introvertive anhedonia, a high level of cognitive disorganisation, and an average level of unusual experiences. Thus, on the basis of their dominant schizotypal feature, these clusters were referred to as the 'Unusual Experiences', 'Cognitive Disorganisation', 'Introvertive Anhedonia', and 'Low Schizotypy' subgroups, respectively. DISCUSSION Cluster analytic results revealed the existence of four distinct subgroups of individuals, characterised by different patterns of scores on the three indices of schizotypy. These findings not only provide evidence for the multidimensional nature of schizotypal traits, but also attest to the ecological validity of the factorial structure of schizotypy. The first subgroup consisted of individuals with relatively low scores on all three subscales. This cluster may therefore represent a general lack of vulnerability for schizophrenic symptomatology. In the second cluster, however, individuals exhibited predominantly high scores on the unusual experiences subscale of the OLIFE; this cluster was therefore referred to as the 'Unusual Experiences' subgroup. Given that the unusual experiences subscale measures primarily positive schizotypal features, such as magical thinking and perceptual distortions, this subgroup could be aligned with the reality distortion syndrome of positive schizophrenic symptoms (Liddle, 1987). For instance, magical thinking feasibly is a milder version of delusional thoughts and perceptual distortions, an attenuated version of hallucinations.
Schizotypal trait dimensions
881 ........<>........
Cluster 1 (n=19)
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Cluster 2 (n=27)
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Cluster 3 (n=18)
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Cluster 4 (n=5)
3 n
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Fig. 1. Mean z-scorefor cluster on schizotypyscales.
In the third cluster individuals were distinguished by high scores on the cognitive disorganisation subscale. They also displayed moderately high scores on both the unusual experiences and introvertive anhedonia subscales of the OLIFE. On the basis of the predominant characteristics, the cluster was referred to as the 'Cognitive Disorganisation' subgroup. The configuration of cognitive disorganisation, in combination with more general positive traits and certain negative schizotypal features, suggests an alignment with the Disorganisation syndrome of schizophrenia (Liddle, 1987). The Unusual Experiences and Cognitive Disorganisation subgroups therefore share, to some extent, the ideational and perceptual aspects of schizotypy in a manner analogous to the overlap in hallucinations and delusions between the Reality Distortion and Disorganisation syndromes of schizophrenia (Williams, 1996). Cluster 4 was differentiated by individuals with very high scores on the introvertive anhedonia subscale; they also displayed moderately high scores on the cognitive disorganisation subscale of the OLIFE. This cluster was therefore termed the 'Introvertive Anhedonia' subgroup. Given that this subgroup is characterised by purely negative schizotypal traits, it appears to correspond with the psychomotor poverty syndrome of negative schizophrenic symptoms (Liddle, 1987). However, given the small ( n - 5 ) number of participants in the Introvertive Anhedonia subgroup, such an interpretation must be regarded as somewhat tentative. We nevertheless gain some confidence from the fact that participants' high scores on the Introvertive Anhedonia subscale showed only moderate variation (SD = 0.56). Although the Unusual Experiences, Cognitive Disorganisation and lntrovertive Anhedonia subgroups were defined predominantly by elevated scores on distinct schizotypal traits, it is nonetheless worth noting the degree of overlap between these subgroups in non-defining traits. The contribution of ideational and perceptual disturbances (which define the Unusual Experiences subgroup) to the Cognitive Disorganisation subgroup was mentioned above in the context of parallelism between these schizotypal subgroups and the schizophrenic syndromes of Reality Distortion and Disorganisation, respectively. In the schizophrenic Disorganisation syndrome, the presence of thought
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C.M. Loughland and L. M. Williams
disorder in addition to hallucinations and delusions is indicative of a particularly severe form of the disorder (Bilder, Mukherjee, Rieder & Pandurangi, 1995). A less straightforward finding is the contribution of cognitive disorganisation to the Introvertive Anhedonia subgroup, and of introvertive anhedonia to the Cognitive Disorganisation subgroup. Such overlap between these subgroups might be explicable in terms of the existence of social anxiety in each cluster; perhaps different forms of social anxiety. Kretschmer (1925, cited in Mishlove & Chapman, 1985) differentiated characteristics of social anxiety in terms of hyperaesthetic and anaesthetic features. According to Kretschmer (1925, cited in Mishlove & Chapman, p. 385), hyperaesthetic anxiety results from a "hypersensitivity to potential rejection, humiliation and shame". Anaesthetic anxiety, on the other hand, refers to social withdrawal that results from a personal indifference to people. Given this distinction, it is possible that the Cognitive Disorganisation subgroup is delineated by a non-clinical manifestation of the hyperaesthetic form of social anxiety, as reflected in the fundamentally positive characteristics of this subgroup, namely, the personality characteristics of suspiciousness and paranoia. This particular subgroup may manifest social anxiety as a secondary consequence of their mild positive symptomatology and bizarre ideas. That is, they may deliberately withdraw socially as a way of coping with or minimising their suspicious and paranoid thoughts and beliefs. The Introvertive Anhedonia subgroup, on the other hand, might be more precisely delineated by a non-clinical anaesthetic form of social anxiety, as reflected in the personality characteristics of schizoid withdrawal and anhedonia in this subgroup. That is, this subgroups' social anxiety might represent a primary schizotypal feature that is part of the anhedonic personality type. Thus, social anxiety as a schizotypal feature may contribute to both the Cognitive Disorganisation and Introvertive Anhedonia subgroups, but in qualitatively different ways. Our findings are not only consistent with previous cluster analytic results (Williams, 1994), but also correspond broadly with those from factor analyses. In terms of cluster analytic research, the results support Williams' finding that individuals may be grouped into four schizotypal trait subgroups. The Low Schizotypy, Unusual Experiences, Introvertive Anhedonia, and Cognitive Disorganisation subgroups of the present study are directly comparable to Williams' Clusters 1 to 4, respectively. As in Williams' study, the configuration of schizotypal features in each of our subgroups suggested a phenomenological correspondence with Liddle's (1987) schizophrenic syndromes of reality distortion, disorganisation, and psychomotor poverty. Our findings therefore, not only confirm the multidimensional nature of schizotypy in terms of how it exists within individuals, but also add weight to the notion that Liddle's schizophrenic syndromes represent p r i m a r y syndromes that have some basis in normal personality, rather than reflect secondary consequences of the schizophrenic process. In relation to factor analytic findings, the results attest to the ecological validity of the tridimensional structure of schizotypy proposed by Claridge et al. (1996). That is, our cluster typology indicates that Claridge et al.'s factors of Aberrant Perceptions and Beliefs (or Unusual Experiences), Cognitive Disorganisation and Introvertive Anhedonia, respectively, are broadly manifested within individuals. To investigate the association between schizotypal trait subgroups in the vulnerability to distinct syndromes of psychosis, there is a need to validate the present typology in a clinical sample. As a means of externally validating such an association, future studies might also determine whether corresponding schizotypal and schizophrenic subgroups perform similarly on cognitive tasks. The present authors are currently undertaking such a study. The value of establishing the validity of schizotypal subgroups as precursors of particular schizophrenic syndromes is that subsequent research into schizophrenic processes may be conducted confidently on non-clinical samples. In this way the confounding effects of drug therapy, institutionalisation and separation from the community are avoided.
REFERENCES Barkley,R. A. (1991). The ecologicalvalidityof laboratoryand analogueassessmentmethodsof ADHD symptoms.Journal of Abnormal Child Psychology, 19, 149 178.
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Bentall, R. P., Claridge, G. S. & Slade, P. D. (1989). The multidimensional nature of schizotypal traits: A factor analytic study with normal subjects. British Journal of Clinical Psychology, 28, 363-375. Bilder, R. M., Mukherjee, S., Rieder, R. O. & Pandurangi, A. K. (1995). Symptomatic and Neuropsychological correlates of defect states. Schizophrenia Bulletin, 11,409-4 19. Carpenter, W. T., Heinrichs, D. W. & Wagman, A. M. I. (1988). Deficit and nondeficit forms of schizophrenia: The concept. American Journal of Psychiatry, 145, 578-583. Chapman, L. J., Chapman, J. P. & Raulin, M. L. (1976). Scales for physical and social anhedonia. Journal of Abnormal Psychology, 85, 374-382. Chapman, L. J., Chapman, J. P. & Raulin, M. L. (1978). Body-image aberration in schizophrenia. Journal of Abnormal Psychology, 87, 399-407. Claridge, G. (1985). Origins of mental illness. Oxford: Blackwell. Claridge, G., McCreery, C., Mason, O., Bentall, R., Boyle, G., Slade, P. & Popplewell, D. (1996). The factor structure of 'schizotypal' traits: A large replication study. British Journal of Clinical Psychology, 35, 103-115. Eckblad, M. & Chapman, L. J. (1983). Magical ideation as a indicator of schizotypy. Journal of Consulting and Clinical Psychology, 51, 215-225. Eckblad, M. L., Chapman, L. J., Chapman, J. P. & Mishlove, M. (1982). The revised social anhedonia scale. Unpublished test. University of Wisconsin-Madison. Farmer, A. E., McGuffin, P. & Spitznagel, E. L. (1983). Heterogeneity in schizophrenia. A cluster-analytic approach. Psychiatry Research, 8, 1 12. Gur, R. E., Mozley, P. D., Resnick, S. M., Levick, S., Erwin, R., Saykin, A. J. & Gur, R. C. (1991). Relations among clinical scales in schizophrenia. American Journal of Psychiatry, 148, 472-478. Hair, J. F., Anderson, R. E., Tatham, R. L. & Black, W. C. (1992). Multivariate data analysis with readings (3rd ed.). New York: MacMillan. Kelley, M. P. & Coursey, R. D. (1992). Factor structure of schizotypy scales. Personali o, and Individual Differences, 13, 723 731. Kendler, K. S. & Hewitt, J. (1992). The structure of self-report schizotypy in twins. Journal of Personality Disorders, 6, 1 17. Liddle, P. F. (1987). Schizophrenic syndromes, cognitive performance, and neurological dysfunction. Psychological Medicine, 17, 49 57. Mason, O., Claridge, G. & Jackson, M. (1995). New scales for the assessment of schizotypy. Personality and Individual Differences, 18, 7 13. Mishlove, M. & Chapman, L. J. (1985). Social anhedonia in the prediction of psychosis proneness. Journal of Abnormal Psycholoyy, 94, 384-396. Morrison, R. L., Bellack, A. S., Wixted, J. T. & Mueser, K. T. (1990). Positive and negative symptoms in schizophrenia. A cluster-analytic approach. Journal of Nervous and Mental Disease, 178, 377 384. Muntaner, C., Garcia-Sevilla, L., Fernandez, A. & Torrubia, R. (1988). Personality dimensions, schizotypal and borderline personality traits and psychosis proneness. Personality and Individual Differences, 9, 257-268. Raine, A. & Allbutt, J. (1989). Factors of schizoid personality. British Journal of Clinical Psychology, 28, 31-40. Sauer, H., Geider, F. J., Binkert, M., Reitz, C. & Schroder, J. (1991). Is chronic schizophrenia heterogenous? Bioloyical Psychiatry, 29, 661S. Van der Does, A. J. W., Linszen, D. H., Dingemans, P. M., Nugter, M. A. & Scholte, W. F. (1993). A dimensional and categorical approach to the symptomatology of recent-onset schizophrenia. Journal of Nervous and Mental Disease, 181, 744-749. Williams, L. M. (1994). The multidimensional nature of schizotypy traits: A cluster analytic study. Personality and Individual Differences, 16, 103-112. Williams, L. M. (1996). Cognitive inhibition and schizophrenic symptoms subgroups. Schizophrenia Bulletin, 22, 139 151.