Link between urban life and schizophrenia was initially suggested in 1939 (Mental disorders in urban areas: an ecological study of schizophrenia and other psychoses. Faris, R. E. L.; Dunham, H. W. Oxford, England: Univ. Chicago Press. (1939). xxxviii 270 pp). This association has been consistently replicated since then by various studies exploring the links in terms of population density, population size, the timing of exposure (urban birth, upbringing or onset of illness), disease nature( broad or narrow schizophrenia). All these have shown that living in urban environment increases the risk of developing schizophrenia. Social drift and differential service utilisation theories have been refuted to a larger extent. A dose response relationship ie higher incidence of schizophrenia seen with higher degree of exposure to urban environment in terms of duration and severity also has been established (Pedersen CB, Mortensen PB, 2001). See March et al,2008 and Kelly et al 2010 for recent review of the literature.
This meta analysis by Evangelos Vassos et al is an attempt to provide a pooled relative risk for schizophrenia relative to a continuous variable of the level of exposure to the risk factor (urbanicity index).
All relevant studies published until Dec 2o11 that met the inclusion criteria were included.
Inclusion criteria : a. Comparing incidence rates (IRs) of schizophrenia between different groups of urbanicity exposure b. Measuring of urbanization should be done prior to the earliest stage of the disease process (at birth or under 15 years of age ie to avoid bias due to selective migration of individuals with psychosis as we know that the effect of urbanization operate before the time of illness onset). c.Study populations were national or representative cohorts and cases were identified from national registers of psychiatric disorders.
Urbanicity was measured as a continuous variable and that any individual in the population was assigned to a point along a uniformly distributed urbanicity scale [U(0, 1)].Meta-analysis was performed to produce a single estimate of the effect size of urbanicity as a risk factor of schizophrenia.
4 studies that met the strict inclusion criteria. Studies were conducted between 1974 and 2007 in 6 Western industrialized countries with predominantly European populations and comprised a total of 46 820 cases with psychosis. 4 additional studies met a broader criteria for inclusion. A summary method as well as a novel simulation method was were used to summarise the data.
Included studies: Lewis, 1992, Marcelis,1998, Harrison,2003 Denmark ( Pedersen et al 2001.Mortensen et al 1999 (overlapping cohort studies)
Estimated pooled OR from the simulation method for schizophrenia in the 4 studies that met strict criteria was 2.37 (95% confidence interval [CI] = 2.01–2.81).
In all 4 studies, as urbaniciy index increased, the Incidence Rate (IR) and Incidence Rate Ratio (IRR ie against rural ) also increased.
Conclusions: The incidence of schizophrenia increases almost linearly with increase in urbanicity. This observation remained stable irrespective of the outcome measure (narrow schizophrenia or broad psychosis), the method of measuring urban environment (population size, place of residence or population density), and the time of exposure (birth and upbringing or onset of illness).
Implications: This could inform the service organisation in urbanised countries. Urbanicity is a risk marker only, it does not imply causation.Authors raise the need for studies in developing countries with different sociodemographic structures, to understand better the causes of this association and to make universal use of urbanicity as risk factor for schizophrenia.
How might urbanicity increase the risk?
Explanations include differences in individual characteristics (eg, different levels of migration or family history), selective migration, exposure to infections, exposure to pollutants, diet, and the social environment. Neighbourhood effects, social fragmentation and deprivation, and other differences between life in cities and rural areas have been found to explain better the association of urbanicity with psychosis than individual differences.
Summary of the article:
Meta-analysis of the association of urbanicity with schizophrenia. Vassos E, Pedersen CB, Murray RM, Collier DA, Lewis CM. Schizophr Bull. 2012 Nov;38(6):1118-23