Delusions in the general population: A systematic review with emphasis on methodology

Schizophrenia Research xxx (xxxx) xxx

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Delusions in the general population: A systematic review with emphasis on methodology Søren Esben Rytter Heilskov a, *, Annick Urfer-Parnas a, Julie Nordgaard a, b a b

Mental Health Center Amager, Gl. Kongevej 33, 1610, Copenhagen V, Denmark Institute of Clinical Medicine, University of Copenhagen, Blegdamsvej 9, Copenhagen, Denmark

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 July 2019 Received in revised form 18 October 2019 Accepted 20 October 2019 Available online xxx

The presence of delusions is considered a key feature of psychosis, but despite the psychopathological centrality of the concept of delusion, its definition and comprehension is a matter of continuing debate. In recent years studies showing that delusions are common in the general population have accumulated and challenged the way we perceive psychotic illness. In this systematic review, we examine the basis of the psychosis continuum-hypothesis, by reviewing a representative section of the original literature that report measures of delusional ideation in the general population, focusing specifically on methodology. Three online databases were systematically searched for relevant studies. After applying criteria of inclusion and exclusion, 17 articles were included for comprehensive review. Estimates of the distribution of delusions in the general population vary substantially, as does the mode of assessment. The methodology relies with few exceptions exclusively on self-report and fully structured interview by lay person. We conclude that measures of delusions in the general population should be interpreted cautiously due to inherent difficulties in methodology. Hypothesizing a continuum of delusion between normality and full-blown psychosis is deemed premature based on the reviewed studies. © 2019 Elsevier B.V. All rights reserved.

Keywords: Psychosis Psychosis continuum Self-rated Delusion Review Psychopathology General population

Contents 1. 2. 3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Studies using a version of the composite international diagnostic interview (CIDI) (Andrews and Peters, 1998) . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Studies using the Peters et al. Delusions Inventory-21 (PDI-21) (Peters et al., 1999, 2004) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Studies using the community assessment of psychic experiences (CAPE) (Stefanis et al., 2002) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Studies using the mini international neuropsychiatric interview (MINI) (Sheehan et al., 1998) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Other studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Delusions

are

considered

a

key

feature

of

psychosis,

* Corresponding author. Gl. Kongevej 33, 1610, Copenhagen V, Denmark. E-mail address: [email protected] (S.E.R. Heilskov).

00 00 00 00 00 00 00 00 00 00 00 00 00

distinguishing normality from pathology, but recent reports suggesting that delusions are common in the general population are challenging the traditional categorical perception of psychotic illness. In the current diagnostic systems, the presence of delusions remains one of the main criteria for psychotic disorders (American

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Psychiatric Association, 2013; World Health Organization, 1992). In the DSM-III to IV delusion is defined as an epistemic error, i.e. incorrect inference about external reality. The definition of the DSM-5 section on schizophrenia is more relaxed, defining delusion as “fixed beliefs that are not amenable to change in the light of conflicting evidence” (American Psychiatric Association, 2013). Unfortunately, “false believes” and “fixed ideas” are widely present in general population. For these reasons, defining delusion and grasping what it is, has been an headache of psychiatric scholars since the beginning of the 20th Century and still is today (Bovet and Parnas, 1993; Cermolacce et al., 2010; Jaspers, 1963). In a nutshell, delusion cannot be defined on the basis of isolated propositional content, disconnected from the subjects experiences and a wider context (Parnas and Henriksen, 2016; Stanghellini and Raballo, 2015). Thus, despite the psychopathological centrality of the concept of delusion, its definition and comprehension is a matter of continuing debate. Recently, a quite different view has been gaining momentum in psychiatric research, namely the notion that psychotic symptoms such as delusions and hallucinations are widely prevalent in the general population e constituting a psychosis continuum. According to this theory, delusions are not specific markers of mental illness, but rather in continuum with normal human experience. In a much cited meta-analysis, van Os et al. (2009) report a median prevalence of subclinical psychotic experience in the general population of 5,3%, and argue that psychotic symptoms are expressed at various levels as a result of different genetic and environmental interactions. Accordingly, psychosis occur when enough symptoms have accumulated and a certain threshold is reached. In an updated meta-analysis the same authors report a median prevalence of psychotic experiences (not distinguishing between clinical and subclinical) in the general population of 7,2% (Linscott and van Os, 2013), and a recent world-wide survey including 31,261 respondents from 18 countries found a lifetime prevalence of psychotic experiences in the general population of 5,8% (McGrath et al., 2015). Psychotic symptoms, found in a population of otherwise healthy subjects with no psychiatric history, are often labelled as psychoticlike experiences. However, the concept of psychotic likeexperiences is not well defined, and there is no consensus, e.g. on whether psychotic like-experiences are distinguished from true psychotic symptoms based on phenomenological differences in the experiences or on the fact that they are not associated with need of care (Lee et al., 2016; Stanghellini et al., 2012). One the same note, when dealing with a continuum of psychotic experiences, a distinction can be made between two concepts: On one hand, the notion that psychotic experiences exist on a continuum within a spectrum of mental illness (namely the schizophrenia spectrum), referred to as the quasi-dimensional model, and on the other hand, the fully dimensional model, presented in psychosis continuum hypothesis, considering full-blown psychotic symptoms found in psychiatric patients and psychotic-like experiences found abundantly in healthy subject from the general population as points on the same continuum (Claridge, 1994; David, 2010; van Os et al., 2009). Whereas the quasi-dimensional model is to a large extend compatible with the categorical view of psychotic illness that permeates the current diagnostic systems, the psychosis continuumhypothesis seems to represent a radical change in the conceptualization of psychotic symptoms. Proposing such a change requires solid empirical evidence embedded in a rigorous conceptual and methodological analysis. In this study, we examine the basis of the psychosis continuum-hypothesis, by reviewing a representative section of the original literature that report measures of delusional ideation in the general population. The main focus is on methodology; the mode of assessment and psychometric tools used, and

the results are discussed in relation to the epistemological issues inherent in the assessment of psychotic symptoms. 2. Methods Three online databases; PubMed, PsycINFO and Embase were searched to identify articles published between 1st of January 1950 and 1st of May 2018 containing the following terms: “psychosis continuum” or “extended psychosis phenotype” or “continuum hypothesis” or “continuum of psychosis” or “subclinical” AND “delusions” or “delusion” or “delusional ideation” After excluding doublets, the combined searches yielded a total of 235 publications. All abstracts were read, and the following criteria of inclusion end exclusion were applied to select papers relevant for this review: a) Only studies presenting original data in English were included. b) Studies had to report an estimate of the distribution of delusions/delusional ideation/delusion-like belief in the general population. c) Subjects had to be (roughly) representative of the general population (e.g. not enrolled in a psychiatric setting) with a mean-age of 18 years or above. d) Studies had to include at least 100 subjects. Of 235 publications, 17 met the criteria listed above and were included for comprehensive review. See Fig. 1 for full search text and PRISMA flowchart. 3. Results The 17 publications included in this review present results from 14 unique observational studies. Reported rates of delusional ideation in the general population vary between 3,0% endorsing at least one “delusional-like experience” as reported by Saha et al. (2011b) to 91% endorsing at least one “delusion-like belief” as reported by Preti et al. (2012). Study characteristics and results are depicted in Table 1. The studies used different assessment methods, and will in the following be reported according to the psychometric tool used. 3.1. Studies using a version of the composite international diagnostic interview (CIDI) (Andrews and Peters, 1998) Seven studies assessed delusion with a version of the CIDI, which is a structured diagnostic interview developed for epidemiological research in mental health. Studies on the psychometric properties of the CIDI found excellent reliability and adequate validity (Andrews and Peters, 1998), except in assessing psychotic symptoms (Cooper et al., 1998), and for this reason, the psychosis section was left out in the CIDI version 3. Accordingly, psychotic symptoms are assessed with considerable variability in the different CIDI versions. In the National Comorbidity Survey (Murphy et al., 2010) a modified version of the CIDI, with 13 items inquiring about classic psychotic experiences, was administered by trained interviews. Answers were recorded as “yes” or “no”, without room for interpretation by interviewers or follow-up by psychiatrist. In the NEMESIS-1 study (Smeets et al., 2013; van Os et al., 2000) delusions were assessed first by a lay interviewer using the CIDI 1.1 psychosis module consisting of 17 questions. Subjects with responses rated as either “true psychiatric symptom” or “plausible”

Please cite this article as: Heilskov, S.E.R et al., Delusions in the general population: A systematic review with emphasis on methodology, Schizophrenia Research, https://doi.org/10.1016/j.schres.2019.10.043

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Fig. 1. Search strategy.

were contacted over the phone by a psychiatrist who re-evaluated the answers using questions from the SCID-III-R. Responses rated by lay interviewer as either “not clinically relevant”, a result of somatic illness/drug ingestion or “no symptom”, were not followed up. The authors do not report the number of psychiatric reinterviews or the positive predictive value of a CIDI rating of “true psychiatric symptom” compared to psychiatrist rating. Of the total sample, 4,2% had at least one “true psychiatric symptom”, and were on this basis denoted as cases of “Clinical Psychosis”. In the NEMESIS-2 study (van Nierop et al., 2012) psychotic symptoms were assessed with an add-on instrument to the CIDI version 3, which is largely comparable to the one used in NEMESIS1 (Smeets et al., 2013; van Os et al., 2000). Though in the NEMESIS2, the lay interviewers made no judgment regarding the clinical relevance of the symptom, but simply recorded “yes”, “no” or “refuse”. All subjects with at least one positive psychosis-rating were then contacted over the telephone by an “experienced clinician at the level of psychologist or psychiatrist” and re-interviewed similarly as in the NEMESIS-1, and symptoms were re-labelled as either “true positive” or “false positive”. Positive predictive values for self-reported delusions ranged from 18% to 48,5%. 43 participants (0,7%) meet the criteria for a diagnosis of “psychotic disorder”, of whom only 22 had been available for re-interview. Loch et al. (2011) report findings from the Sao Paulo Epidemiological Catchment Area Study where subjects were interviewed at home by trained interviewers using the CIDI version 1.1. Delusions were assessed in the same manner as in the NEMESIS-1 study, but

there was no subsequent follow-up in terms of identifying true cases of delusion. Based exclusively on the CIDI-ratings, 31 of 1464 individuals were assigned an ICD-10 diagnosis of Non-Affective Psychosis. In the Australian National Survey of Mental Health (Saha et al., 2011a, 2011b) another add-on to the CIDI 3.0 was used, consisting of three screen items concerning delusional-like experiences, each followed by a probe question. Interviews were conducted by trained interviewers from the Australian Bureau of Statistics. Two papers (Smeets et al., 2012; Spauwen et al., 2003) present results from the Munich Early Developmental Stages of Psychopathology study, a prospective cohort where young subjects were assessed with a modified version of the CIDI version 1.2 at baseline with 3 follow-ups over a 10-year period. Assessment was conducted by clinical psychologists. At two follow ups the section of the CIDI 1.2 inquiring about delusions and hallucinations was administered. Interviewers were allowed to follow up with clinical probe questions. 3.2. Studies using the Peters et al. Delusions Inventory-21 (PDI-21) (Peters et al., 1999, 2004) Two studies (Fonseca-Pedrero et al., 2012; Preti et al., 2012) used the PDI-21, a brief self-administered questionnaire constructed to measure delusional ideation in the general population. Questions are derived from the Present State Examination, but have been toned down by adding an as if in order to “capture its normal

Please cite this article as: Heilskov, S.E.R et al., Delusions in the general population: A systematic review with emphasis on methodology, Schizophrenia Research, https://doi.org/10.1016/j.schres.2019.10.043

4

van Os et al. (2000)

The Netherlands Mental Health Stratified, random sample Survey and Incidence Study-1 of the Dutch population (NEMESIS-1)

Spauwen et al. (2003)

Munich Early Developmental Stages of Psychopathology study The Greek Birth Cohort

Random, representative sample of young adults (mean age 21,8 years) Cohort of Greek adolescents Community Assessment of Self-administered Psychic Experiences (CAPE) questionnaire

Murphy et al. 2010

The National Comorbidity Survey

Loch et al. (2011)

The Sao Paulo Epidemiological Catchment Area Study

Stratified multistage area probability sample of citizens from 48 states in USA Age-stratified selection of inhabitant of two boroughs of the City of Sao Paulo Random sample of the British population Random sample of private dwellings in Australia

CIDI version 3.0

Undergraduate students (mean age ¼ 24,4 years) from the University of Cagliagri Incidental sample of Spanish college students (mean age ¼ 20,3 years) Random, representative sample of young adults (mean age ¼ 26,6 years) Stratified, random sample of the Dutch population

PDI-21

Self-administered questionnaire

No

504

PDI-21

Self-administered questionnaire

No

650

Average PDI-21 total score: 4,30 (SD: 2,78)

No

2210

11,2% report having had at least one “delusion” in the past 5 years

Pechey and Halligan, 2011

Saha et al., 2011a þb

Australian National Survey of Mental Health and Wellbeing

Preti et al. (2012)

Fonseca-Pedrero et al. (2012)

Psychometric tool

Method of assessment

Psychiatrist involved in assessment?

Main findings

Cohort/study name

Stefanis et al. (2004)

Population

n

Authors

Partly, see results 7075

3,3% of subjects had at least one “psychiatrist-rated delusion”

No

2548

No

3500

Modified version of the CIDI Fully structured interview version 1.0 by trained lay interviewers

No

5893

Lifetime cumulative incidence of at least one “delusional experience” was 15,7% The proportions endorsing at least one item were 65% for paranoia, 36% for first-rank symptoms and 24% for grandiosity. Endorsement rate of delusional items ranged from 1,09 to 12,88%

CIDI version 1.1

Fully structured interview by trained lay interviewers

No

1464

Cardiff Beliefs Questionnaire

No Fully structured telephone interview conducted by market research company Fully structured interview by No trained lay interviewer

1000

CIDI version 1.1

Fully structured interview by trained lay interviewers, with partial follow-up by psychiatrist. Modified version of the CIDI Fully structured interview by version 1.2 clinical psychologists

Modified version of the CIDI Fully structured interview by version 1.2 clinical psychologists

8773

Smeets et al. (2012)

Munich Early Developmental Stages of Psychopathology study

van Nierop et al. (2012)

Partly, see results 6646 Modified version of the CIDI Fully structured interview by version 3.3 trained lay interviewers, with partial follow-up by psychiatrist or psychologist Partly, see results Baseline: 7075 The Netherlands Mental Health Stratified, random sample CIDI version 1.1 Fully structured interview by Year one: 5616 Survey and Incidence Study-1 of the Dutch population trained lay interviewers, with Year three: 4845 partial (telephone) follow-up by psychiatrist, psychologist or trainee psychiatrist Random sample of Swedish CAPE No 1012 Internet based citizens self-administered drawn from a voluntarily questionnaire registered panel Incidental sample of CAPE Internet based selfNo 2880 the Dutch population administered questionnaire

Smeets et al. (2013)

Ziermans, 2013

Wigman et al. (2017)

The Netherlands Mental Health Survey and Incidence Study-2

4,4% of subjects (no cases excluded) had at least one “clinically relevant delusion” 38% of subjects strongly endorsed one or more “delusional-like belief” 3,0% endorsed both screen and probe item for at least one “delusional-like experience” 91% report at least one “delusion-like belief”. Average PDI-21 total score: 5,8 (SD: 4,0)

Lifetime prevalence of specific “delusions” assessed by lay interviewer varied from 0,1e6% The proportion of subject with “any delusion” was 13,2% at baseline, 3,8% one year after and 2,0% 3 years after baseline Endorsement rate of “psychotic-like experiences” with delusional content ranged from 4,0 to 88,8% Endorsement rate of delusional experiences (“positive psychotic experiences”) ranged from 0,6 to 81,6%

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Table 1 Study characteristics and main findings of the included studies.

7,4% reported to have delusions 809 No Fully structured interview by trained psychologists MINI
nez et al. Guerrero-Jime (2018)

Pignon et al. (2018)

The French Mental Health in the Non-probabilistic General Population survey quota-sampling, representative of the French population P The GRANAD P study Stratified random sample of the Spanish population

Fully structured interview by Partly, see results 38694 The Mini International Neuropsychiatric Interview trained nurses and psychologist (MINI)

20,5% has at least one “delusional symptom”

S.E.R. Heilskov et al. / Schizophrenia Research xxx (xxxx) xxx

5

equivalent” (Peters et al., 1999). For each positive answer, subjects are asked to rate the experiences on three dimensions; belief strength, preoccupation and distress, on a five-point Likert-scale. The authors behind the PDI-21 report that the tool has more than adequate internal consistency and test-retest reliability, and its construct validity has been establish both by comparing it to a similar self-report tool, and by observing that a heterogenous group of deluded patients (n ¼ 33) endorse 16 of the 21 items significantly more often than healthy subjects (Peters et al., 2004). Neither of the two studies included in this review used any form of follow-up by clinician to validate the delusional content of responses. 3.3. Studies using the community assessment of psychic experiences (CAPE) (Stefanis et al., 2002) Three studies (Stefanis et al., 2004; Wigman et al., 2017; Ziermans, 2013) used the CAPE e a 40-item self-report instrument based on the PDI-21, but with some modification; mainly the addition on two items concerning hallucinatory experiences and 22 items concerning negative/depressive symptoms. Neither of the three studies used any form of follow-up. 3.4. Studies using the mini international neuropsychiatric interview (MINI) (Sheehan et al., 1998) The MINI is a fully structured interview with a median time of completion of 15 min. Assessment of delusions consists of five questions inquiring about various “delusional symptoms” and symptoms are recorded as present or not-present. Comparing the clinician-rated diagnoses of “current psychotic disorder” obtained in MINI-interviews with a clinicians-administered SCID-gold standard, the authors behind the MINI found the positive predictive values “acceptable, but in the lower range” (Sheehan et al., 1998). Pignon et al. (2018) present results from a survey, in which 38.694 subjects were interviewed by 3-day trained nurses or psychologists using the MINI. 1075 diagnoses (2,8%) of “characterized psychotic disorder” were assigned based on the MINI interview with subsequent confirmation by a senior psychiatrist. The authors do not report what this confirmation consisted of or the number of subjects who underwent re-evaluation by a psychiatrist. P
nez et al., 2017, 2018) In the GRANAD P study (Guerrero-Jime subjects were interview by trained psychologists using the MINI. Having delusions was defined as at least one affirmative answer to one of the five “delusional symptom”-items, with estimates based exclusively on the MINI-ratings. 3.5. Other studies Pechey and Halligan (2011) used a self-report instrument, The Cardiff Beliefs Questionnaire in which 17 questions about “delusion-like beliefs” are embedded among 29 questions addressing various beliefs regarding paranormal, religious, political and science-related themes. Assessment was carried out by a market research company and all interviews were done over telephone. 4. Discussion This is the first review of the empirical studies on delusions in the general population that focuses on methodology. Our findings show that reports of delusions in the general population rely on either pure self-report or fully structured interviews with little or no follow-up by a relevant clinician. This approach is questionable. But before commencing our discussion of the methodological challenges, it’s worth reflecting a bit on the distinction between psychotic symptoms vs. psychotic-like experiences, and delusions

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Table 2 Terms used to describe outcome in relation to the applied psychometric tools. Psychometric tool

Terms used to describe outcome

Examples of questions used in assessment

Composite International Diagnostic Interview (CIDI)

“Delusions” (Loch et al., 2011; Smeets et al., 2012, 2013; van Nierop et al., 2012; van Os et al., 2000) “Schneiderian Delusion” or “Paranoid Delusion” (Murphy et al., 2010) “Delusional experience” (Spauwen et al., 2003)

“In the past 12 month have you had the feeling that people were too interested in you?” (Cooper et al., 1998) “Have you ever felt that your thoughts were being directly interfered with or controlled by another person” “Do you have any special powers that most people lack?” (Saha et al., 2011b)

Peters et al. Delusions Inventory-21 (PDI-21)

Community Assessment of Psychic Experiences (CAPE) The Mini International Neuropsychiatric Interview (MINI)

The Cardiff Beliefs Questionnaire

“Delusional-like experiences” (Saha et al., 2011a, 2011b) “Delusional experiences” (Fonseca-Pedrero et al., 2012) “Delusion-like beliefs” (Preti et al., 2012)

Item 1: “Do you ever feel as if people seem to drop hints about you or say things with a double meaning” Item 14: “Do you feel that you have sinned more than the average person?” Item 19: “Have your thoughts ever been so vivid that you were worried other people would hear them” (Peters et al., 2004) Same as in the PDI-21

“Positive psychotic experiences” (Stefanis et al., 2004; Wigman et al., 2017) “Psychotic-like experiences” (Ziermans, 2013) “Psychotic symptoms” and “Delusions”
nez et al., 2017, 2018; (Guerrero-Jime Pignon et al. (2018); Poulton et al., 2000)

“Delusion-like beliefs” (Pechey and Halligan, 2011)

vs. delusion-like experiences. Table 2 shows the various terms used to describe measures of delusional ideation in relation to the psychometric tool used in assessment, along with examples of questions from each tool. Several different terms are used, also among studies using the same psychometric tool, and whether findings are termed “delusions” or “delusion-like experience” does not seem to reflect differences in mode of assessment. Based on the same observations, one study finds “positive psychotic experiences” and another finds “psychotic-like experiences”, etc. In sum, our findings confirm what others have argued as one of the main challenges in the field of research into subclinical psychotic symptoms: The lack of consensus on definitions and use of terminology (Lee et al., 2016; Schultze-Lutter et al., 2011). Hence, interpreting the results in relation to a strict distinction between delusions and delusion-like experiences is not deemed feasible, as this distinction is not reflected in empirical differences in the reviewed studies. Returning to the methodological issue: In order to extract a reliable estimate of the prevalence of delusions in the general population, the studies need to be comparable. As we have shown, the studies differ in how delusions are assessed e both in terms of extent and level of observation. Beginning with extent; since a common way to report measures of delusions in the general population is the proportion of subjects that endorse at least one delusional belief, it is not irrelevant whether assessment consists of three questions (Saha et al. (2011b) finding that 3,0% has at least one “delusion-like experience”) or twenty-one questions (Preti et al. (2012) finding that 91% has at least one “delusion-like belief”). This is reflected in a remarkably wide range of estimates, as shown in Table 1. Regarding the level of observation; five studies base their estimates exclusively on information gained through a self-administered questionnaires (PDI, CAPE) and nine studies use various fully structured interview-tools administered by interviewers with a professional background ranging from clinical psychologist (Smeets et al., 2012; Spauwen et al., 2003) to telephone marketing research-assistant (Pechey and Halligan, 2011). Three studies have follow-up interviews by a psychiatrist or clinical psychologist in order to identify the true delusions, but only for

Item 1: “Have you ever believed that people were spying on you, or that someone was plotting against you, or trying to hurt you?” Item 4: “Have you ever believed that you were being sent special messages through the TV, radio, or newspaper, or that a person you did not personally know were particularly interested in you” Item 5: “Have you relatives or friends ever considered any of your beliefs strange or unusual?” “You are dead and/or do not exist” “People you know disguise themselves as other to manipulate or influence you” (Pechey and Halligan, 2011)

selected subgroups, and follow-up either consist of a telephone interview with a clinician using question from the SCID (van Nierop et al., 2012; van Os et al., 2000), or is simply not accounted for (Pignon et al., 2018). Though the present sample does not allow for meaningful statistical analysis, it could be argued that there is at trend towards lower rates reported in the studies that employ follow up. Not surprisingly, just as the number of questions asked influence the number of subjects that test positive, so does how the question is asked and who interprets the answers. As previously mentioned the concept of delusion is notoriously difficult to force into a context-independent definition, and thus it is surprising that none of the studies reviewed here question or discuss whether the concept delusion has any meaning in itself. It is assumed that delusions exist in the general population and are accessible for standard measurement similarly to blood pressure or albuminuria, regardless of the various cultural and religious beliefs that permeates the specific setting. Interpreting a PDI-score as a measure of delusion, means accepting that diverse human experiences such as worrying that your partner may be unfaithful (item 13) and feeling that other people may be able to hear your thoughts (item 19) are equal representatives for this concept, and can be used as such without further attempt to understand the context in which they arise. It could be argued that an approach that focuses on the content of beliefs tends to overlook an essential aspect of the concept of delusion (Cermolacce et al., 2010). Explained briefly, psychosis cannot be assessed solely on basis of mental content (the “what” of experience) e as implied in many self-rating questionnaires e but requires knowledge of the structure of the experience (the “how” of experience) and, not least of the context in which it occurs. This is well exemplified in Schneider’s report of the patient who found indisputable evidence of the imminent apocalypse when seeing a dog lifting its paw on the street (Schneider, 1959): The belief that the world is about to end is not delusional per se (environmental concerns, religious beliefs etc.), but is revealed as such by virtue of the peculiar form of experience described by the patient. In other words, identifying delusion is only possible when the proposition is

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connected to the experience of the subject and a wider context (Stanghellini and Raballo, 2015). Self-report tools and fully-structured interviews have some obvious advantages in epidemiological research: Symptoms can be assessed relatively fast and inexpensively with high interobserver agreement. This allows studies to be applied in large populations and strengthens reliability, thus enhancing representativity and giving high statistical power. Furthermore, tools like the PDI-21 and the CAPE attempt to put the experiences into context by adding dimensions of belief strength, preoccupation and distress, thus contributing with important information on how the experiences impact on the respondents’ life. However, the value of such qualifying measures still depends on the validity of the main outcome (delusions) with which they are associated. Several empirical studies have demonstrated the problems inherent to assessing psychotic symptoms using self-report or fully-structured interviews: Schultze-Lutter et al. (2014) compared various self-report instrument (among these the PDI) to clinicians rating in a psychiatric setting and found only a weak correlation between self-rated delusion-like experience and clinicians rating of attenuated psychotic symptoms and/or psychotic symptoms. They conclude that psychotic-like experiences “at least when assessed with the PDI […] cannot be regarded as valid estimate of clinician-rated psychotic symptoms, even if additional qualifiers are taken into account” (Schultze-Lutter et al., 2014). Nordgaard et al. (2018) tested a 13-item questionnaire of psychosis-like-symptoms in a general population setting. Following up on a section of those with affirmative answers using semi-structured interview by nurses trained in psychopathology, they found that 82,5% of subjects positive of self-rated psychosislike symptoms were false positives. Studies validating the psychosis module of the CIDI found that while it tends to underdiagnose schizophrenia in a clinical population (Cooper et al., 1998) the trend is reversed when used in the general population (Kendler et al., 1996). Kendler et al. (1996) tested the CIDI version 1.1 in a large US community sample and compared the diagnoses generated with those assigned after reinterview by a clinician. They concluded: “Standard structured psychiatric interviews, analysed by computer, are a questionable method of case-detection for psychotic illness in the general population and generate an unacceptable high proportion of false positives” (Kendler et al., 1996). Employing follow-up by a psychiatrist for selected subgroups seems like a reasonable way to improve validity, but a precondition for using this method (putting aside whether a brief telephone interview with a psychiatrist using questions from the SCID represents a psychopathological gold standard) is that the instrument used to select subjects are reliable screening-tools for psychotic symptoms, which is e at best e unproven. Likewise, taking into account various dimensions of experiences in assessment is a step forward, but for now, their utility is limited by the poor validity of the construct they are supposed to nuance. 4.1. Limitations This study has some limitations: In order to be included in this review, studies must present data on at least 100 subjects from the general populations. This is in line with previous large metaanalysis on the subject (van Os et al., 2009), and aims at ensuring that results can be interpreted as representative for the general population. There is a risk though, that studies employing a more qualitative approach in smaller samples are excluded. Also, this study focuses on delusions and the their role in the psychosis continuum-hypothesis. The role of hallucinations and other psychotic symptoms in the psychosis continuum-hypothesis is an import field of research, but lies beyond the scope of this review.

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4.2. Conclusion Based on the studies reviewed here it can be concluded that estimates of delusions in the general population vary considerably. Assessments rely, with few exceptions, exclusively on self-report and fully structured interview by lay person; an approach that has been shown as flawed when it comes to assessing psychotic symptoms. Furthermore, the field of research is marked by terminological inconsistency and lack of strict conceptual distinctions. In summary, measures of delusions in the general population should be interpreted very cautiously, and hypothesising a continuum of delusion between normality and full-blown psychosis is deemed premature based on the studies reviewed here. Future studies, applying qualified psychopathological expertise in a general population-setting, are warranted, and their interpretation should take into account the conceptual and clinical complexity of this phenomenon. Acknowledgement We want to thank Professor Josef Parnas for reading the manuscript and sharing his thoughts. References American Psychiatric Association, 2013. Diagnostic and Statistical Manual of Mental Disorders : DSM-5. x. Andrews, G., Peters, L., 1998. The psychometric properties of the composite international diagnostic interview. Soc. Psychiatry Psychiatr. Epidemiol. 33 (2), 80e88. Bovet, P., Parnas, J., 1993. Schizophrenic delusions: a phenomenological approach. Schizophr. Bull. 19 (3), 579e597. Cermolacce, M., Sass, L., Parnas, J., 2010. What is bizarre in bizarre delusions? A critical review. Schizophr. Bull. 36 (4), 667e679. Claridge, G., 1994. Single indicator of risk for schizophrenia: probable fact or likely myth? Schizophr. Bull. 20 (1), 151e168. Cooper, L., Peters, L., Andrews, G., 1998. Validity of the composite international diagnostic interview (CIDI) psychosis module in a psychiatric setting. J. Psychiatr. Res. 32 (6), 361e368. David, A.S., 2010. Why we need more debate on whether psychotic symptoms lie on a continuum with normality. Psychol. Med. 40 (12), 1935e1942. Fonseca-Pedrero, E., Paino, M., Santaren-Rosell, M., Lemos-Giraldez, S., Muniz, J., 2012. Psychometric properties of the Peters et al Delusions Inventory 21 in college students. Compr. Psychiatr. 53 (6), 893e899.
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