Psychotic Experiences and Psychotic Disorders at Age 18 in Relation to Psychotic Experiences at Age 12 in a Longitudinal Population-Based Cohort Study

Psychotic experiences exist as a continuum within the population in terms of the degree of conviction, preoccupation, frequency, and number of different phenomena experienced (1–3). Their impact varies widely in terms of associated distress, effect on functioning, and help-seeking behavior. In an attempt to understand more about the etiology of psychotic disorders, many studies have focused either on psychotic experiences within general population samples (4–12) or on high-risk samples of individuals who present to clinical services with symptoms deemed indicative of a high risk for developing a psychotic illness (13–17).

The advantages of studying general population samples rather than high-risk samples are that they are likely to capture individuals earlier in the trajectory of a disorder and that these samples include individuals with psychotic phenomena who might never present to clinical services. However, unlike studies of high-risk samples, most investigations of general population samples use self-report measures or structured interviews akin to self-report measuresto assess psychotic experiences. These instruments overestimate the occurrence of psychotic experiences because, unlike semistructured interviews that most closely approximate clinical assessments, they do not allow for cross-examination to ensure that the experiences described are psychotic. For example, the prevalence of psychotic phenomena in childhood ranges from 5% for psychotic experiences elicited from semistructured interviews (8, 18) to 60% for self-reported experiences (7). This variance has important implications for interpreting the results from studies examining the proportion of those with psychotic experiences who make the transition to psychotic disorders; indeed, transition rates across general population studies vary widely (4, 5, 9, 19), with substantial heterogeneity (I²=92%) reported in a recent meta-analysis (20).

Few general population studies have used repeated semistructured interviews to examine the transition from psychotic experiences to psychotic disorders over time (4, 5) or to identify groups at highest risk of transition—akin to at-risk mental states (sometimes referred to as prodromal syndromes) in clinical samples. Only two small studies (21, 22, with samples of 212 and 58, respectively) have examined the prevalence of such syndromes in nonclinical samples. The prevalence of at-risk mental states in a large, representative, population-based cohort sample and their relationship to prior measures of psychotic experiences earlier in childhood is not known.

In this study, we examined the development of psychotic experiences elicited from repeated semistructured interviews at ages 12 and 18 in a large epidemiologically defined birth cohort. Our primary aims were to describe the prevalence and clinical features, including help-seeking behavior, of psychotic experiences, psychotic disorders, and at-risk mental states at age 18. We also examined the relationship between psychotic experiences at age 12 and the development of psychotic outcomes at age 18.

Method

Sample

We examined data from 4,724 young adults from the Avon Longitudinal Study of Parents and Children (ALSPAC) cohort who were assessed with the Psychosis-Like Symptom Interview (18) at age 18 (mean=17.8 years, SD=0.38). The initial cohort consisted of 14,062 births (www.alspac.bris.ac.uk), and 9,919 young adults were invited to interview when they reached age 18 (see 23 and 24 for further details). All participants provided written consent. Ethical approval was obtained from the ALSPAC Law and Ethics Committee and the local research ethics committees.

Measures

Psychotic experiences at age 18.

The semistructured Psychosis-Like Symptom Interview draws on principles of standardized clinical examination developed for the Schedule for Clinical Assessment in Neuropsychiatry (SCAN). An introductory section on unusual experiences comprising six questions on derealization, depersonalization, self-unfamiliarity, dysmorphophobia, partial object perception, and other nonspecific perceptual abnormalities is followed by 11 core questions eliciting key psychotic experiences occurring since age 12: hallucinations (visual and auditory), delusions (spied on, persecution, thoughts read, reference, control, and grandiosity), and experiences of thought interference (broadcasting, insertion, and withdrawal). Any unspecified delusions elicited from the interviewees were also rated. Cross-questioning was used to establish the presence of symptoms, and coding followed glossary definitions and rating rules for SCAN.

The interviewers were psychology graduates trained in assessment using the SCAN psychosis section and the Psychosis-Like Symptom Interview, and they were blind to previous assessments. The interviewers rated experiences as not present, suspected, or definitely psychotic. Unclear responses after probing were always “rated down,” and symptoms were rated as definite only when a clear example was provided. At regular intervals, a psychiatrist rated samples of recorded interviews to ensure that the interviewers were rating experiences correctly.

If the interviewers rated experiences as suspected or definitely psychotic, they also asked about frequency; impact on affect, social function, and educational/occupational function; help seeking; age at onset; and attributions (see Appendix A in the data supplement that accompanies the online edition of this article).

Reliability.

For interrater reliability, the interviewers recorded audio interviews at three time points, approximately 6 months apart, across the clinic duration (75 interviews in total). The average kappa value of psychotic experiences was 0.83, with no evidence of differences across time. Test-retest reliability was assessed using 162 individuals reinterviewed after approximately 47 days (kappa=0.76, SE=0.078), 46 of whom were reinterviewed by the same interviewer (kappa=0.86, SE=0.136).

Psychotic disorders and at-risk mental states at age 18.

We classified individuals as having a psychotic disorder if they reported definite psychotic experiences not attributable to the effects of sleep or fever that had occurred at least once per month over the previous 6 months and either caused severe distress, had a markedly negative impact on social or occupational function, or led to help seeking. At-risk mental states were assessed by relating Psychosis-Like Symptom Interview data to the Structured Interview for Prodromal Syndromes (SIPS) definitions of prodromal syndromes (25, 26) (see Appendix B in the online data supplement).

Psychotic experiences at age 12.

The Psychosis-Like Symptom Interview was also used at age 12 (18) to identify the following: 1) definite psychotic experience not attributed to sleep or fever, 2) suspected psychotic experience not attributed to sleep or fever, 3) suspected or definite psychotic experience attributed to sleep or fever, 4) unusual experiences such as depersonalization, and 5) self-reported endorsement of psychotic experience not supported by interviewer ratings.

Sociodemographic characteristics.

Data on sex, parental social class, maternal marital status, financial difficulty, housing type, and parental education were collected from birth records and parental questionnaires (see Appendix A in the online data supplement).

Statistical Analysis

Logistic regression was used to calculate odds ratios and 95% confidence intervals (95% CI) for psychosis outcomes at age 18 in relation to background characteristics and experiences at age 12. For ordinal independent variables, the odds ratios are per category increase. Analyses were conducted using Stata, version 11 (StataCorp, College Station, Tex.).

For multiple imputation of missing data, we used flexible additive imputation models as implemented in the aregImpute function in the R statistical package, with estimates averaged over 100 imputed data sets. We included 90 auxiliary variables that could inform psychotic experience or missingness status, making missingness at random conditional on these factors a plausible assumption. Imputation was performed for the subsample who participated in either of the interviews at age 12 and age 18 (N=7,456) as well as for the whole ALSPAC sample with adequate data for imputation (N=14,229).

Results

Psychotic Experiences at Age 18

Of 4,724 young adults assessed with the Psychosis-Like Symptom Interview when they were 18 years old, 433 (9.2%; 95% CI=8.4–10.0) were rated as having either suspected (N=203 [4.3%; 95% CI=3.7–4.9]) or definite (N=230 [4.9%; 95% CI=4.3–5.5]) psychotic experiences since age 12 (Figure 1). Of these, 272 (62.8%) had only one experience, 91 (21.0%) had two, 38 (8.8%) had three, and 32 (7.4%) had four or more different types of experiences. The most common experiences were auditory hallucinations (5.4% suspected or definitely psychotic), visual hallucinations (4.2%), and beliefs about being spied on (1.7%) (see Table S1 in the online data supplement). Of those with definite psychotic experiences, 60 (1.3% of sample) were incident in the previous year.

Individuals were more likely to have suspected or definite psychotic experiences if they were female or had higher levels of socioeconomic disadvantage (p<0.001) (Table 1). Of 433 young adults with suspected or definite psychotic experiences at age 18, 58 (13.4%) had experiences attributed to effects of sleep or fever, and these were recoded as nonpsychotic; 375 individuals (7.9% of those interviewed) had experiences not attributable to these states (Figure 1). Psychotic experiences attributable to sleep or fever were much less likely to occur frequently or impair function compared with the experiences not attributed to these causes (see Table S2 in the online data supplement), although 12.5% of individuals described them as very distressing.

Continuity of Psychotic Experiences From Age 12 to Age 18

Of 6,796 children who were interviewed at age 12, 4,060 (59.7%) participated in the interview at age 18. Of these, 100 (2.5%) had suspected or definite psychotic experiences at both time points, 370 (9.1%) had experiences at age 12 only, 190 (4.7%) had experiences at age 18 only, and 3,400 (83.7%) had them at neither time point. The 100 individuals with psychotic experiences at both time points represent 21.3% of those with psychotic experiences at age 12 (i.e., the persistent symptom group), with 78.7% having only transient psychotic experiences (Figure 2).

Psychotic Disorders at Age 18

Of 367 young adults with suspected or definite psychotic experiences who responded to the questions on help seeking, 47 (12.8%) sought professional help. Help seeking did not differ by sex or social class, although the power to examine this was limited (see Table S3 in the online data supplement). Sixteen young adults (0.3% of the sample) were taking medication for their psychotic experiences. Individuals with definite psychotic experiences were approximately 10 times more likely to have sought professional help than those with suspected ones (24% compared with 3%; odds ratio=9.6, 95% CI=3.7–25.0; p<0.001). Individuals reporting a greater number of different experiences were more likely to have sought professional help (range, 0–10; odds ratio per experience, 1.76; 95% CI=1.42–2.17; p<0.001).

Young adults seeking professional help were more likely than non-seekers to rate experiences as distressing (57% compared with 13%), more frequent (22% compared with 7%), and as having a very negative impact on social function (36% compared with 3%) and occupational function (38% compared with 2%) (p<0.001 in all cases).

Seventy-nine young adults (1.7% of sample, 95% CI=1.3–2.1) met criteria for a psychotic disorder at age 18. Of these, only 41 (51.9%) had sought help. Auditory or visual hallucinations were present in 77% of individuals with a psychotic disorder. All these individuals also met diagnostic criteria for psychotic disorders as defined in both DSM-IV and ICD-10, given that they had regular psychotic phenomena that were causing them severe distress or substantially impaired functioning.

Prediction of Psychotic Disorders at Age 18

The odds of having a psychotic disorder at age 18 were higher in those with suspected psychotic experiences at age 12 (odds ratio=5.6, 95% CI=2.6–12.1) and especially in those with definite psychotic experiences at age 12 (odds ratio=12.7, 95% CI=6.2–26.1) (Table 2). The risk of psychotic disorders was also substantially higher among those with psychotic experiences at age 12 that were attributed to the effects of sleep or fever (odds ratio=5.9, 95% CI=1.7–20.5).

The odds of having a psychotic disorder were also three times higher in those who had unusual experiences such as depersonalization but not psychotic experiences at age 12 compared with those who had no such experiences. The pattern of associations was similar when we examined a broader outcome of definite psychotic experiences at age 18, and even individuals with self-reported experiences at age 12 (that were not rated by the interviewer as psychotic) had twice the odds of developing definite psychotic experiences at 18 compared with those who had no self-reported experiences at age 12.

The positive predictive value for psychotic disorders at age 18 increased, while the sensitivity decreased, as the criteria for psychotic experiences at age 12 narrowed (Table 3). Approximately 75% of those with a psychotic disorder at age 18 reported some abnormality at age 12, although the positive predictive value using this cutoff was only 3%. The positive predictive value was highest for those with nonattributed psychotic experiences occurring on a daily basis at age 12 (22.5%), with a sensitivity of 14.5% (see Table S4 in the online data supplement), although these estimates were based on very few individuals. Positive predictive values were similar for hallucinations, delusions, and experiences of thought interference (see Table S10 in the online data supplement).

At-Risk Mental States

In addition to the 79 individuals with a psychotic disorder, 35 others (0.7% of sample, 95% CI=0.5–1.0) met SIPS criteria for an at-risk mental state (20 with brief intermittent prodromal syndrome [0.4%] and 20 with attenuated positive symptom prodromal syndrome [0.4%]; five individuals met criteria for both). Of these, only one person had sought professional help.

Multiple Imputation

Participants with missing outcomes at age 18 were more likely to be male, to come from socioeconomically deprived backgrounds, and to be more impaired on cognitive, behavioral, and psychological measures at younger ages. Prevalence estimates for psychotic experiences were similar in the imputed and complete case samples. The results for associations between experiences at age 12 and outcomes at age 18 were also similar, although effect sizes were smaller in the imputed samples (see Tables S5–S7 in the online data supplement).

Discussion

Psychotic Outcomes at Age 18

Approximately 5% of individuals in our cohort had a definite psychotic experience between the ages of 12 and 18. The 1-year incidence (1.3%) and cumulative risk (4.9%) of definite psychotic experiences in this cohort are close to the lower ends of the interquartile ranges for incidence and prevalence estimates from a recent meta-analysis of all studies examining psychotic experiences in nonclinical samples (2). This is likely to be because our estimates are based on semistructured interviews rather than the structured interviews or self-report questionnaires used by most studies in the review, and indeed, a meta-analysis of studies examining the prevalence of psychotic phenomena attributed 54% of study heterogeneity (95% CI=27%–73%) to methodological factors (27).

Relatively few other studies have used semistructured interviews to assess psychotic experiences in general population samples for comparison. In another study that used a semistructured interview based on the Psychosis-Like Symptom Interview, the prevalence of psychotic experiences at age 12 was approximately 5%, similar to what we found in our study (8). However, in a smaller study of young adolescents, 20% were rated as having psychotic symptoms (21). In the Netherlands Mental Health Survey and Incidence Study (NEMESIS) and the Early Developmental Stages of Psychopathology (EDSP) study, psychotic experiences were assessed using a structured interview (4, 5), although interviewers could ask additional probing questions. In both studies, the prevalence of psychotic experiences was approximately 18%. The prevalence of delusional items in the EDSP study was substantially higher than in our study, whereas the prevalence of hallucinations (approximately 5%) was similar (28), indicating that the main source of variation in the prevalence of psychotic experiences is the criteria used for defining delusional beliefs.

Psychotic experiences were more common in young women in our study. Although schizophrenia is more common in men (29), studies of psychotic experiences in nonclinical population-based samples have found rather inconsistent evidence for sex differences in the presence of these phenomena (2, 6, 30). The greater prevalence of psychotic experiences in individuals from more deprived socioeconomic backgrounds is consistent with findings in schizophrenia (31) and with theories linking socioeconomic adversity and social defeat to dopaminergic abnormalities (32), although this is perhaps not surprising given that most health outcomes show similar socioeconomic patterning.

In our study, 1.7% of 18-year-olds met criteria for a psychotic disorder. Approximately 75% of these would have been identified if the interview had been restricted to hallucinations only, suggesting that where resources are limited, assessing hallucinatory items only might be an efficient method for identifying a substantial proportion of individuals with a psychotic disorder in the general population.

When we applied criteria from the SIPS, a further 0.7% met criteria for at-risk mental states, consistent with the low incidence of psychotic disorders in young people (29, 33) and with the prevalence of at-risk mental states from a telephone survey of 58 young people in Switzerland (22). Furthermore, although 8% of 212 adolescents in Ireland met SIPS criteria for a prodromal syndrome, 0.9% met the most recent Comprehensive Assessment of At-Risk Mental States criteria (21). This highlights the difficulty of applying criteria that are to some extent arbitrary to identify clinically relevant groups that might benefit from targeted interventions. Assessments from semistructured interviews such as the SCAN, the modified Composite International Diagnostic Interview, the SIPS, and the Comprehensive Assessment of At-Risk Mental States rely on subjective decisions as to whether criteria (however defined) are met, but they are nevertheless the gold standard for assessing psychotic phenomena. Routinely used cutoffs to categorize the severity of psychotic experiences (4, 5, 8, 9, 18, 34) are also inevitably somewhat arbitrary.

Instruments such as the SIPS or the Comprehensive Assessment of At-Risk Mental States, which assess at-risk mental states in clinical samples, have not been validated in general population samples, and there are limitations in deriving SIPS-defined syndromes from the Psychosis-Like Symptom Interview, as the assessments are not wholly comparable. Whether at-risk mental states defined in this way index a greater risk for transition to a psychotic disorder compared with simpler measures, such as the presence of psychotic experiences in general population samples, is unknown, but the transient nature of at-risk mental states in clinical samples (35) and the low levels of help-seeking behavior observed in our study suggest that attempts to identify high-risk individuals through health care providers is unlikely to be a successful strategy.

The strength of this study is that we used data from a large and well-characterized birth cohort, with repeated semistructured assessments of psychotic experiences covering a period of particular interest in the development of psychotic disorders. One of the main limitations, however, as with most longitudinal studies, is the possibility of selection bias caused by the substantial attrition over time. For example, approximately 60% of individuals who participated in the psychosis assessment at age 12 also completed the interview at age 18, but this represents only 35% of the original cohort of live births recruited into the study.

However, sensitivity analyses using multiple imputation show that selection bias is unlikely to have substantively affected the estimates. Indeed, these estimates are likely to be conservative compared with the true strength of the association between psychotic experiences at age 12 and age 18, because the models undersample the group with persistent experiences between these ages. This is in keeping with our multivariate analysis of missingness, which indicates that data from participants at higher risk of psychosis because of sociodemographic, behavioral, and psychological characteristics were more likely to be missing, and with simulation studies of attrition in this cohort (36).

Clinical Characteristics

Approximately 24% of those individuals with definite psychotic experiences at age 18 sought professional help, in keeping with estimates reported in other studies (4, 37). Most definitions of high-risk mental states, such as those derived from the SIPS, have been driven by research in clinical samples, in which help seeking is a universal characteristic. However, within general population samples, help-seeking behavior is not present for many individuals who nevertheless meet criteria for a psychotic disorder or who present a high risk of developing one. In our study, approximately 50% of those meeting criteria for a psychotic disorder, and 95% of those meeting SIPS criteria for a prodrome, had not sought clinical help for their experiences, indicating a high level of unmet need.

Prediction of Psychosis

The presence of a psychotic disorder at age 18 was more common not only in those individuals who were rated as having suspected or definite psychotic experiences at age 12, but also in those who reported unusual experiences such as (nonpsychotic) perceptual disturbances or who reported phenomena that were not rated as suspected psychotic experiences after follow-up questioning during the semistructured interview (“false positives”). In fact, approximately 80% of those with a psychotic disorder by age 18 responded affirmatively to self-report questions at age 12, indicating that the majority of individuals with outcomes of possible clinical relevance in early adulthood appeared to have had unusual experiences from as far back as childhood.

Not unexpectedly, psychotic phenomena increase the risk of developing a psychotic disorder (4, 5, 9, 12, 19, 20, 38). However, only studies that use both a structured and semistructured approach can establish whether false positive experiences are of pathological significance. Our findings are consistent with and extend those from other studies examining psychosis-related outcomes of such experiences (39, 40) and provide strong support for the hypothesis that psychosis exists as an extended phenotype (27, 41).

Implications of Identifying High-Risk Individuals

An Israeli study reported low positive predictive values (1%–4%) for self-reported psychotic phenomena predicting hospital admissions for a psychotic disorder (38). In our study, individuals with definite psychotic experiences at age 12, particularly when frequent, were most likely to have a psychotic disorder by age 18. However, even for those experiencing the most frequent symptoms at age 12, the positive predictive value was only about 20%, which fell rapidly when less-frequent or suspected phenomena were included. Screening young adolescents to identify a group with daily psychotic experiences for closer monitoring (approximately 1% of our cohort at age 12) might be viewed as an attractive means of potentially reducing the incidence and impact of psychotic outcomes in young adults. However, even if interventions that were 100% effective were implemented, this strategy would prevent less than 15% of psychotic disorders at age 18.

The use of biomarkers and information on neurocognitive (42, 43) or neuroimaging deficits (44) may help increase prediction for transition (45), but in the absence of strong and specific predictors for clinical psychoses, the trade-off between increasing positive predictive value and decreasing sensitivity is likely to restrict attempts at targeted prevention strategies (46).

Conclusions

Our findings demonstrate a continuum of psychotic experiences from as early as age 12 and affecting up to 10% of 18-year-olds. The positive predictive value of early-adolescent psychotic experiences (approximately 80% of which are transient) predicting adult psychotic disorders is too low to make the risk-benefit ratio of current interventions a feasible option. Although psychotic disorders and high-risk states, as assessed in this study, were relatively uncommon at age 18, the high levels of associated distress and impairment indicate an important unmet need for care, given that most individuals had not yet sought help from clinical services for these problems.