Exploring the causal relationships between neighbourhood characteristics and severe mental illness: A systematic review

Purpose Severe mental illness (SMI) is an umbrella term for chronic psychiatric disorders associated with significant functional impairment, including schizophrenia and other psychotic disorders, bipolar disorders, and major depression. Neighbourhood characteristics have emerged as potentially modifiable risk factors for SMI. While associations between neighbourhood-level characteristics and SMI are well-documented, fewer studies have explored their causal relationship. This systematic review synthesises the evidence on the causal relationship between physical and social neighbourhood environments, and SMI. Methods We searched four electronic databases (Medline, EMBASE, PsycINFO, Web of Science) and hand-searched reference lists of included studies. Title/abstract screening, full-text screening, quality assessments, and data extraction were conducted by two independent reviewers. Due to heterogeneity in methodologies, findings were narratively synthesised. Results Thirteen studies were included, encompassing both social and physical characteristics. All studies were rated as high quality. A range of causal methods were employed, including Mendelian randomisation (n = 7), quasi-experiments (n = 4), family-based designs (n = 1), and causal mediation (n = 2). There was some evidence for an association between ethnic density and SMI (n = 1), and weak evidence for a link between social cohesion and SMI (n = 1). Evidence for causal associations between SMI and deprivation (n = 3) or air pollution (n = 7) was mixed, and population density (n = 2) showed weaker, inconclusive associations with SMI. Conclusion Although causal claims remain tentative due to heterogeneity in definitions, methods, and populations, the evidence points towards the neighbourhood context as a contributory factor in the development of SMI, highlighting the need to shift attention towards community-level public health interventions addressing neighbourhood-level determinants.