Background Despite increased ethnic diversity in more economically developed countries it is unclear whether residential concentration of ethnic minority people (ethnic density) is detrimental or protective for mental health. This is the first systematic review and meta-analysis covering the international literature to assess ethnic density associations with mental health outcomes.

Methods We systematically searched Medline, PsychInfo, Sociological Abstracts, Web of Science from inception to March 31 st, 2016, for observational studies using search terms for the following study outcomes: Depression, anxiety and the common mental disorders, suicide, suicidality, psychotic experiences and psychosis. We obtained additional data from study authors. We conducted random effects meta-analysis taking into account clustering of estimates within datasets. Meta-regression assessed heterogeneity in studies due to ethnicity, country, generation and area-level deprivation. This review is registered with PROSPERO CRD42014012992.

Results Out of 2288 studies identified 41 met inclusion criteria. 12 studies provided data for meta-analysis and meta-regression. In the meta-analyses, we found a large reduction in relative odds of psychotic experiences (OR: 0.82 (95% CI: 0.76–0.89) (nine estimates)) and suicidal ideation (OR: 0.88 (95% CI: 0.79–0.98) (ten estimates)) for each 10 percentage-point increase in own ethnic density. For common mental disorders, depression and anxiety, associations were indicative of protective effects of own ethnic density however results were not statistically significant.

Conclusion The findings support consistent protective ethnic density associations across countries and racial/ethnic minority populations as well as mental health outcomes. This is indicative of the importance of the social environment in patterning detrimental mental health outcomes in marginalised and excluded population groups.