Chapter_2F - 47 Chapter Two The Continuity Model of...

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Chapter Two The Continuity Model of Psychosis The current project examines delusional thinking, but unlike many of the previous studies of delusional thinking, it does not examine differences between deluded patients and healthy controls. Instead, participants were drawn from the normal population and were placed in a high or low delusional ideation group, based on scores taken from a measure of delusional ideation in the normal population (the Peters et al. Delusions Inventory (PDI); Peters, Day & Garety, 1996). In each experiment reported here, high and low scoring groups were compared on reasoning tasks. The first section of the following chapter provides a theoretical rationale for the study of delusional thinking in the normal population. The second section then describes the PDI. Psychosis as a Continuum There is now a considerable body of evidence for the notion that psychosis falls at the extreme end of a spectrum which also includes normal, healthy behaviour. This theory is at odds with Kraepelin (see Bentall, 2003), who believed that psychosis (dementia praecox) was utterly distinct from normal behaviour, and is also largely at odds with the position of Jaspers (1914). However, there are two perspectives on the nature of the continuity of psychosis. Firstly, the clinical view put forward by Meehl (1962; 1990), is that certain vulnerable individuals possess an inherited proneness to schizophrenic symptoms. The severity of these symptoms may be measurable along a dimension, but a dimension which is discontinuous with normality (Lenzenweger & Korfine, 1995). Meehl (1990) posits that up to 10 per cent of the population may have such a vulnerability, 47
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which is underpinned by a central nervous system anomaly (schizotaxia), and of these, around 50 per cent are expected to develop schizophrenia. Hence, many individuals will carry the latent liability for schizophrenia but not all of these will develop the disease. Claridge and Beech (1995) describe Meehl’s conceptualisation of schizotypy as ‘quasi- dimensional’, in that it represents a disease continuum rather than a full personality dimension. Taking the continuity theory a step further, Claridge and Beech view schizophrenia as continuous with normality, a view often referred to as the ‘individual differences approach’. The severest forms of the illness exist at the extreme end of the dimension, with normality as its polar opposite. Less severe forms of psychotic behaviour can be found at points along the continuum. Importantly, Claridge and colleagues’ argument in favour of the individual differences view incorporates the notion that schizophrenia and the other forms of psychosis are related, and represent less severe points along an illness continuum. The evidence for such a view comes from three sources: family studies, reports of psychotic-like experiences in healthy individuals and psychometric data. The evidence from family studies will be reviewed first. Taylor (1992) carried out a review of family studies. He found high incidence of affective
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Chapter_2F - 47 Chapter Two The Continuity Model of...

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