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Introduction
Elevated rates of psychotic illness among ethnic minority populations have been consistently demonstrated but never fully explained (Cantor-Graae & Selten, 2005; Fearon et al. 2006). In recent years a growing body of evidence suggests this cannot be dismissed as merely an artefact of misdiagnosis and a series of international comparison studies have ruled out simple genetic explanations (Fearon et al. 2006; Morgan & Hutchinson, 2010; Selten & Cantor-Graae, 2010). Attention has therefore focused on environmental factors, such as the social stress and social isolation associated with an ethnic minority status (March et al. 2008). An ethnic density effect has been proposed whereby the proportion of an ethnic group living in an area is inversely related to the risk of psychosis for members of that group.
This relationship between ethnic density and psychosis was first identified over 70 years ago (Faris & Dunham, 1939) and has since been replicated in other settings for a range of migrant groups (Pickett & Wilkinson, 2008). However, as a recent review concludes, UK studies present a more mixed picture (Fung et al. 2009). The first to address this question looked at the association between hospital admission rates, for schizophrenia, and ethnic density, for a number of migrant groups, at a national and regional health authority level and found no evidence for an effect (Cochrane & Bal, 1988). Boydell and colleagues then looked at ethnic density at ward level using a simple binary definition of ethnicity and found incidence rates for non-whites were significantly higher in areas with a lower non-white population (Boydell et al. 2001). The Aetiology and Ethnicity of Schizophrenia and Other Psychoses study also addressed the question of ethnic density and found some effect using a similar definition of ethnicity to Boydell et al. (Kirkbride et al. 2007b). This study had a larger sample, including 163 black and minority ethnic (BME) participants newly diagnosed with non-affective psychoses, compared with 126 non-white incident cases of schizophrenia in Boydell's study. However, the ethnic density effect failed to reach statistical significance and disappeared completely when a more detailed ethnicity definition was used. A recent study using national survey data also looked at the effect of ethnic density on psychotic symptomatology and...