376 Current Pharmaceutical Design, 2012, 18, 376-385 1873-4286/12 $58.00+.00 © 2012 Bentham Science Publishers Can We Detect Psychotic-like Experiences in the General Population? B. Nelson*, P. Fusar-Poli and A.R. Yung Orygen Youth Health Research Centre, Centre for Youth Mental Health, University of Melbourne, Australia Abstract: The continuum model of psychosis posits that psychotic symptoms are distributed throughout the population, with diagnosable clinical disorder existing at a certain point along this continuum. The total continuum is made up mainly of non-clinical cases with clini- cal cases of psychosis representing only a small proportion of the total extended psychosis phenotype. This paper is a narrative review of studies of psychotic experiences in the general population. The evidence indicates reasonably high prevalence rates of psychotic experi- ences in the general population, substantially higher than the prevalence of psychotic disorders, and that they are associated with in- creased risk of future onset of diagnosable disorder, particularly when the experiences are persistent. Psychotic experiences in the general population share an extensive range of risk factors with schizophrenia and therefore provide a useful phenotype in which to study the ae- tiology of clinical psychosis. Some types of psychotic experiences, such as paranoid ideas, bizarre thinking and perceptual abnormalities, may indicate a greater level of risk for psychotic disorder than other psychotic experiences, such as magical thinking. There is a need for research that further explores the interplay between psychotic experiences and other risk factors (including psychological, environmental, neurocognitive and genetic factors) in the evolution of psychotic disorder, the types of psychotic experiences that are most associated with risk for clinical disorder, the specificity of risk associated with psychotic experiences, and the possible adaptive advantages of these experiences. Keywords: Psychosis, schizophrenia, risk. THE CONTINUUM MODEL OF PSYCHOSIS The notion of psychotic symptoms either being present or ab- sent (the dichotomous model of psychosis) has been replaced over the last several decades with a model of psychotic symptoms exist- ing on a continuum. At a certain point on this continuum clinical disorder is diagnosed. At one end of the continuum lies schizophre- nia and at the other end lies psychotic-like experiences (PLEs) in healthy individuals. Various terms have been used to refer to the sub-clinical manifestations of the psychosis phenotype, including psychosis proneness, psychotic-like experiences and schizotypy [1- 5]. The total continuum is made up mainly of non-clinical cases with clinical cases of psychosis representing only a small propor- tion of the total extended psychosis phenotype[6]. This view of psychotic symptoms can be traced back to Kret- scher [7], who suggested that variations in personality in the general population correspond to expressions of psychosis, and Strauss [8], who described hallucinations and delusions varying continuously in patients along dimensions of conviction, preoccupation and implau- sibility. Since these original ideas were expressed, a substantial body of data has accumulated supporting the notion that psychotic symptoms are experienced not just by patients with psychotic dis- orders but also by a substantial proportion of the general population [9-14]. Estimates of the prevalence and 1-year incidence of psy- chotic symptoms and experiences vary substantially across cohorts and studies. A recent meta-analysis by van Os and colleagues [6] of PLEs in the general population reported a median prevalence rate of about 5%, with an interquartile range of 1.9–14.4%, and a median incidence rate of 3%, with an interquartile range of 1.1–8.6%. In this meta-analysis the authors distinguished between true subclini- cal psychotic experiences, which they estimate has a prevalence of around 8%, and subclinical psychotic symptoms, which are associ- ated with a degree of distress and help-seeking behaviour but do not necessarily constitute clinical psychotic disorder, which they esti- mate has a prevalence of around 4%. These rates are substantially higher than the clinical phenotype of psychotic disorder, consistent with the continuum model. The evidence also indicates the PLEs are more common in adolescence and decline with age [15, 16]. *Address correspondence to this author at the Orygen Youth Health Re- search Centre, Centre for Youth Mental Health, University of Melbourne; Tel: +61 3 9342 2800; Fax: +61 3 9387 3003; E-mail: nelsonb@unimelb.edu.au Two models of continuity have been proposed: the quasi- dimensional model and the fully dimensional model. The quasi- dimensional model conceptualizes PLEs as “form frustes” or vari- ants of disorder, for example, incompletely expressed schizophre- nia. This suggests discontinuity with the normal population. A pro- ponent of this quasi-dimensional model was Meehl [1, 17], who proposed the existence of a “schizoid taxon” which has different phenotypic expressions including schizophrenia. This model also implies that those with PLEs would be at increased risk, or vulner- able to, developing psychotic disorder. For example, the work of Chapman and Chapman suggests that individuals in the normal population with certain features, including PLEs but also social and physical anhedonia, are “psychosis prone” [18-20]. Others also concur with this “forme fruste” or “psychosis prone” idea [12, 21- 29]. The theory is that if the psychosis prone or schizotypal indi- vidual is subject to sufficient psychosocial stress, then onset of psy- chotic disorder may occur, consistent with the stress-vulnerability model [30]. In contrast, the fully dimensional model of psychosis [31, 32] proposes that PLEs are part of personality. For example, Claridge [33] described schizotypal features (including positive psychotic symptoms and anhedonia) as “healthy diversity” and noted that they range from disorder to normal functioning, as described above. This model implies no discontinuity from the normal population and therefore that presence of PLEs do not necessarily indicate risk for future onset of disorder. DOES THE PREVALENCE OF PLEs VARY BY METHOD OF ASSESSMENT? PLEs have been assessed using both questionnaire (i.e., self- report) and interviewer-based methods. It is possible that the former method may lead to overestimation of the prevalence of PLEs as there is no chance to probe and clarify experiences. This has led some authors to argue that PLEs assessed through self-report in epidemiological studies cannot be compared with attenuated psy- chotic symptoms or brief psychotic symptoms as assessed in clini- cal interviews in the clinical high risk paradigm [34]. The literature on this issue is mixed. Konings and colleagues (2006) demonstrated a good correlation between the Community Assessment of Psy- chotic Experiences (CAPE), a self-report measure of positive and negative psychotic experiences, and interviewer-rated PLEs. Simi-