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A Multilevel Longitudinal Study of Variation in Incidence of Psychotic Disorders

Stanley Zammit, PhD; Glyn Lewis, PhD; Jon Rasbash, PhD; Christina Dalman, PhD; Jan-Eric Gustafsson, PhD; Peter Allebeck, PhD

Arch Gen Psychiatry. 2010;67(9):914-922. doi:10.1001/archgenpsychiatry.2010.101

Context  Incidence of schizophrenia and other nonaffective psychoses is greater in urban than rural areas, but the reason is unclear. Few studies have examined whether both individual and neighborhood characteristics can explain this association. Furthermore, as has been shown for ethnicity, the effect of individual characteristics may depend on neighborhood context.

Objectives  To examine (1) whether individual, school, or area characteristics are associated with psychosis and can explain the association with urbanicity and (2) whether effects of individual characteristics on risk of psychosis vary according to school context (reflecting both peer group and neighborhood effects).

Design  Multilevel longitudinal study of all individuals born in Sweden in 1972 and 1977. Diagnoses were identified through linkage with the Swedish National Patient Register until December 31, 2003.

Setting  Population-based.

Participants  A total of 203 829 individuals with data at individual, school, municipality, and county levels.

Main Outcome Measures  Any nonaffective psychosis, including schizophrenia (881 subjects; 0.43% cumulative incidence). For the study of interactions, the outcome was any psychosis (1944 subjects; 0.95% cumulative incidence).

Results  Almost all the variance in risk of nonaffective psychosis was explained by individual-level rather than higher-level variation. An association between urbanicity and nonaffective psychosis was explained by higher-level characteristics, primarily school-level social fragmentation. We observed cross-level interactions between individual- and school-level markers of ethnicity, social fragmentation, and deprivation on risk of developing any psychotic disorder, all with qualitative patterns of interaction.

Conclusions  The association between urbanicity and psychosis appears to be a reflection of increased social fragmentation present within cities. The qualitative interactions observed are consistent with a hypothesis that certain characteristics that define individuals as being different from most other people in their local environment may increase risk of psychosis. These findings have potentially important implications for understanding the etiology of psychotic disorders and for informing social policy.

Author Affiliations: MRC Centre for Neuropsychiatric Genetics and Genomics, Department of Psychological Medicine and Neurology, School of Medicine, Cardiff University, Cardiff, Wales (Dr Zammit); Academic Unit of Psychiatry (Drs Zammit and Lewis) and Centre for Multilevel Modelling (Dr Rasbash), University of Bristol, Bristol, England; Department of Public Health Sciences, Karolinska Institute, Stockholm, Sweden (Drs Dalman and Allebeck); and Department of Education, University of Göteborg, Göteborg, Sweden (Dr Gustafsson).
Deceased.

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