Schizophrenia and the city: A review of literature and prospective study of psychosis and urbanicity in Ireland

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Abstract

Urbanicity has been repeatedly associated with increased incidence of schizophrenia. This article (a) presents results of a prospective study of urbanicity and schizophrenia in Ireland and (b) reviews the literature relating to urbanicity and schizophrenia. We prospectively compared incidence of schizophrenia and other psychoses in urban and rural catchment areas (over 4 years and 7 years, respectively) using face-to-face, DSM-III-R diagnostic interviews. Incidence of schizophrenia in males was higher in urban compared to rural areas, with an age-adjusted incidence rate ratio (IRR) of 1.92 (1.52–2.44) for males and 1.34 (1.00–1.80) for females. Incidence of affective psychosis was lower in urban compared to rural areas for males (IRR 0.48; 0.34–0.67) and females (IRR 0.60; 0.43–0.83). These findings are consistent with the literature, which provides persuasive evidence that risk for schizophrenia increases with urban birth and/or upbringing, especially among males. Register-based studies support this conclusion more consistently than studies using face-to-face diagnostic interviews, the difference being related to power. The mechanism of association is unclear but may relate to biological or social/environmental factors or both, acting considerably before psychotic symptoms manifest. There is a diversity of potential candidates, including air pollution, cannabis and social exclusion. Urbanicity may have a synergistic effect with genetic vulnerability. Future research is likely to focus on the relationship between urbanicity and neural maldevelopment, the possibility of rural protective factors (e.g. social capital, low social fragmentation), urbanicity in developing countries, cultural variables and geographical location, and associations between urbanicity and other disorders (e.g. affective psychosis).

Introduction

Cities are growing at faster rates and in greater numbers than ever before. In 1800, 3% of the world's population lived in cities; in 1900, 14% lived in cities; today, over 50% live in cities (UN-Habitat, 2001). The rate of urban growth is greatest in Africa where, in 2000, 37% of the population lived in cities; by 2020, 48% will live in cities (UN-Habitat, 2003).

Urban residence is associated with increased rates of medical illness (Godfrey and Julien, 2005) including coronary heart disease (Ahmad and Bhopal, 2005), diabetes mellitus (Papoz et al., 1996), metabolic syndrome (Gu et al., 2005), childhood asthma (Dik et al., 2004) and cancers of the esophagus, liver, cervix and female breast (Howe et al., 1993, Kelsey and Horn-Ross, 1993). Urban residence is also associated with increased rates of mental illness (Lewis and Booth, 1992), including childhood psychiatric disorders (Rutter, 1981), adult depression (Sundquist et al., 2004) and adult neurotic disorders (Vazquez Barquero et al., 1982, Blazer et al., 1991). A number of studies have suggested links between schizophrenia and urban birth (Mortensen et al., 1999), urban upbringing (Pedersen and Mortensen, 2001a) and urban residence at time of presentation (Sundquist et al., 2004).

The proposed link between schizophrenia and urbanicity is increasingly important not only because urban populations are growing in size, especially in Africa, Asia and Latin America, but also because many areas, in parallel with urbanization, are also experiencing ‘westernization’ of the epidemiology of both medical (Kelsey and Horn-Ross, 1993, Papoz et al., 1996, Echimane et al., 2000) and psychiatric illness (Eide and Acuda, 1996, Le Grange et al., 1998). If this trend includes schizophrenia there is likely to be a rapid increase in both the incidence and prevalence of schizophrenia in Africa, Asia and Latin America; this may be further compounded by the possibility that the incidence of schizophrenia within cities may also be increasing (Boydell et al., 2003).

These demographic and epidemiological developments highlight the need to develop an epidemiological understanding of the association between urbanicity and schizophrenia, to help identify the biological mechanisms that underpin this association and inform the development of public health programmes for the treatment and, ultimately, prevention of mental illness in urban areas. The aims of this paper are to (a) present the results of a prospective study of urbanicity and psychosis in Ireland; and (b) review the literature relating to urbanicity and schizophrenia.

Section snippets

Prospective study methods

We prospectively identified all individuals with a psychotic episode making their first contact with psychiatric services in (a) a geographically-defined, urban catchment area in South Dublin (1995–1998) with a population density of 15.11 persons per hectare (total population 165,000) (Browne et al., 2000, Whitty et al., 2004) and (b) in two contiguous, geographically-defined, rural catchment areas in North East Ireland (Counties Cavan and Monaghan, 1995–2001) (Scully et al., 2002, Baldwin et

Results of prospective study

One hundred and seventy-one cases were identified in the urban area between 1995 and 1998 and 153 cases were identified in the rural area between 1995 and 2001. All of the cases in the rural area and all but one of the cases in the urban area were ethnically Irish. The risk of schizophrenia in urban males was almost twice that of rural males, with an IRR for males of 1.92 (95% confidence interval, C.I., 1.52–2.44) (Table 1); a similar pattern was present at trend level for females (IRR 1.34,

Discussion of our prospective incidence study

Our prospective incidence study found that the risk of schizophrenia in males residing in urban areas was almost double that of males residing in rural areas. Strengths of this study include the prospective methodology for case ascertainment; comparison of two areas whose principal difference is population density; and use of face-to-face SCID interviews (as opposed to case-registers or case-notes) for diagnosis. This study also addressed the potential confounders of age (indirect

Conclusions

Schizophrenia is associated with urban birth, urban upbringing and urban residence. Evidence from the medical literature about urban excesses of physical illnesses is of limited value in elucidating the biological underpinnings of this relationship, because the basic pathophysiology of schizophrenia is largely unknown.

Future research on urbanicity and schizophrenia might usefully focus on:

  • The relationship between urbanicity and neural maldevelopment;

  • The possible role of confounders (e.g.

Role of funding source

This work was supported by the Stanley Medical Research Institute; the DETECT Programme, Hospitaller Order of St John of God, Ireland; and the Health Service Executive, Ireland.

The study sponsors had no involvement in study design; in the collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the paper for publication.

Contributors

All authors contributed to all stages of this study and have approved the final manuscript.

Conflict of interest

All authors declare that they have no conflicts of interest.

Acknowledgement

This work was supported by the Stanley Medical Research Institute; the DETECT Programme, Hospitaller Order of St John of God, Ireland; and the Health Service Executive, Ireland.

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