for LOP. Copeland et al. estimated an annual inci- dence of 3.0 per 100,000 for DSM-III-R schizophrenia and 16.6 per 100,000 for delusional disorder in those aged 65 years and over [11]. Castle and Murray, applying the same criteria, calculated a rate of 12.6 per 100,000 person-years [9]. Using the broader concept of late paraphrenia, Kay estimated the annual rate for those aged 55 years and older to be 10–15 per 100,000 for males and 20–25 per 100,000 for females [32]. In the general population aged 60 years or older, Van Os et al. showed an 11% increase in ﬁrst admission rates for ICD-9 schizophrenia, paraphre- nia, paranoid states and other non-affective, non-or- ganic psychosis for each 5 year increase in age [64]. Incidence rates rose from 10 per 100,000 person-years in those aged 65 years to 19 per 100,000 in those aged 75 with further progression to 25 per 100,000 in the 90+ age group. Ha¨fner et al. found an annual inci- dence rate of 13 per 100,000 for broadly deﬁned schizophrenia-spectrum disorders within the age range of 50–59 years [18]. These ﬁgures match well with those reported for schizophrenia [20, 51]. Service-based studies, how- ever, are likely to underestimate the frequency of the disorder, and community surveys independent of service use generally ﬁnd higher rates, but researchers have been reluctant to carry out such studies with LOP [68]. Methodological obstacles in conjunction with the supposed infrequency of the disorder may explain this. For example, a diagnosis of LOP is dif- ﬁcult to make using lay interviews. Hence, an alter- native approach is to collect data on the basis of presence of single functional psychotic symptoms without focussing on diagnostic categories. Using this strategy, Christenson and Blazer identiﬁed prevalent paranoid ideation in 4% of a sample of community living elderly [10]. Similar ﬁgures have been found among the non-institutionalized cognitively intact elderly [24], with prevalence rates as high as 10% among the non-demented very old (older than 84 years) [43]. Prevalence rates, however, cannot distinguish between persistence of psychosis with an early onset and psychosis with a true onset in old age. j Risk factors If LOP constitutes a separate phenotype within the schizophrenia spectrum, it is attractive to hypothesize that there will be quantitative or qualitative differ- ences between LOP and EOP with respect to risk factors associated with psychosis such as cannabis use [3, 25, 66], childhood trauma [30, 36], urbanisation [57, 58, 65], schizoid and paranoid premorbid per- sonality [14], neuroticism [24, 34], depressive and other affective symptoms [35, 69] or social isolation [50]. Among the list of putative risk factors for LOP, female sex presents itself as the most prominent one, with women running a two to six times greater risk 289 than men [9, 18, 54], especially if only paranoid states are considered [1, 52]. It seems that men consume their lifetime risk more rapidly than women and are thus hardly found in late-onset pools [20]. A protec- tive role for female sexual hormones in LOP has hence been suggested, as estrogens may decrease the risk in pre-menopausal women by virtue of their antidop- aminergic effects [13, 18, 39, 55]. Another hypothe- sized age-related risk factor may be hearing impairment, particularly in patients aged 60 years and older [28, 46, 50, 62], although recent work sug- gests the effect of hearing impairment is not age speciﬁc [61]. j Research questions and hypotheses The present study calculates incidence rates for psy- chosis as a function of age in the general population of the Netherlands. It was hypothesized that onset of functional psychosis occurs less often in old age than in young and middle age. Second, possible effect modiﬁcation by age of sets of known risk factors for psychosis was assessed, such as depression, neuroti- cism, urbanisation, cannabis use, sex, sensory impairment and family history of psychosis. Method j Sample This study made use of the data from the Netherlands Mental Health Survey and Incidence Study (NEMESIS), a longitudinal study of the prevalence, incidence, course and consequences of psychiatric disorders in the Dutch general population [5, 6, 16]. The study consists of three measurement points with a total duration of three years: at baseline (hereafter: T0), 1 year thereafter (hereafter: T1) and again 2 years after T1 (hereafter: T2). Participants were identiﬁed by means of a multistage, stratiﬁed, random sampling procedure in which 90 municipalities were sampled randomly. In a second step, random selections of addresses from private house- holds were made. Thirdly, the person with the most recent birthday at the moment of selection and aged between 18 years and 64 years received an introductory letter form the Minister of Health. Not included were institutionalized individuals. At baseline, 7,076 subjects participated, which is a response rate of 69.7%. According to the General Health Questionnaire, 12 items (GHQ-12) [17] responders and non-responders did not differ with respect to psychiatric morbidity [5, 6]. At T1, 5,618 subjects (79%) partici- pated, at T2, 4,848 subjects (69%) participated. The local ethics committee approved the study proposal. j Instruments At T0 to T2, the Composite International Diagnostic Interview (CIDI) version 1.1 [56] was administered at home. The CIDI is a structured interview and is designed for the use by trained inter- viewers who are not clinicians. It has satisfactory inter-rater reli- ability [12] and test–retest reliability [67]. Ninety interviewers experienced in systematic data collection administered the inter- view after having received intensive training. In order to assess psychotic symptoms, ratings from the CIDI core psychosis section on delusions (13 items) and hallucinations (4 items) were used