Ethnic isolation and psychosis: re-examining the ethnic density effect

Background. Elevated incidence of psychotic illness has been consistently shown among migrant populations. Ethnic density, the proportion of an ethnic group in a defined area, is cited as one factor with a reduced risk of psychosis where ethnicity is shared. However, UK studies have shown mixed results. We set out to re-examine the ethnic density effect at a greater level of geographic detail than previous studies. Method. Using a large sample of patient records from general practitioners in South East London, we were able to assess neighbourhood factors at the detailed lower super output area level. This comprises, on average, 1500 people compared with around 6000 per ward, the measure used in previous studies. We compared black (Afro-Caribbean) and white psychosis incidence by neighbourhood ethnic density over a 10-year period. Results. We found a clear negative association between ethnic density and psychosis incidence. In neighbourhoods where black people comprised more than 25% of the population, there was no longer a statistically significant ethnic difference in psychosis rates. However, where black people were less well represented, their relative risk increased nearly threefold [odds ratio (OR) 2.88, 95% confidence interval (CI) 1.89-4.39]. Furthermore, incidence rates for black people in the lowest density quintiles were over five times greater than in the most dense quintile (OR 5.24, 95% CI 1.95-14.07). However, at ward level this association was much weaker and no longer statistically significant. Conclusions. Ethnic density is inversely related to psychosis incidence at a detailed local neighbourhood level.