Context and health: a systematic review of natural experiments among migrant populations

Background Studies on contextual effects on health often suffer from compositional bias and selective migration into contexts. Natural experiments among migrants may allow for the causal effect of contexts in generating health inequalities to be examined. We synthesised the evidence on and health from natural experiments among migrant populations. Methods Systematic literature review searching the databases PubMed/MEDLINE, The Cochrane Library, Web of Science, CINAHL and Google Scholar for literature published until October 2022. 5870 studies were screened independently in duplicate using pre-defined criteria for inclusion: quantitative natural experiment methodology, migrant study population, context factor as treatment variable and health or healthcare outcome variable. Synthesis without meta-analysis was performed following data extraction and quality appraisal. Findings The 46 included natural experiment studies provide causal evidence for the negative effects of neighbourhood disadvantage on physical health and mortality, while finding mixed effects on mental health. Studies comparing migrants with those that stayed behind demonstrate the detrimental effects of migration and adverse post-migratory contexts on physical health and mortality, while demonstrating favourable effects for mental health and child health. Natural experiments of policy contexts indicate the negative impacts of restrictive migration and social policies on healthcare utilization, mental health and mortality as well as the positive health effects when restrictions are lifted. Interpretation Natural experiments can serve as powerful tools in reducing bias through self-selection. With careful consideration of causal pathways, results from migration contexts can serve as a magnifying glass for the effects of context for other population groups. Studies demonstrate the negative impacts for health which lie at the nexus of context and health. At the same time, they uncover the potential of health and welfare programs to counteract the disadvantages created by othering processes and promote healthy (post-migratory) contexts. Funding German Science Foundation (FOR: 2928/ GZ: BO5233/1-1). Evidence before this study We searched PubMed/MEDLINE to identify pre-existing reviews on contextual effects on health with the following search terms: ((review[Title/Abstract]) AND (((context[Title]) OR (neighbourhood[Title])) OR (small-area[Title]))) AND (health[Title]). Eight reviews existed and pointed to consistent, but small effects of neighbourhood disadvantage on physical and mental health outcomes, as well as on child and adolescent health. However, these reviews also point to the methodological shortcomings of most studies, which are unable to disentangle compositional from contextual effects. In order to improve causal inference, natural experiments are needed. Natural experiments have previously delivered crucial evidence on the causal effects of public health interventions including suicide prevention, air pollution control, public smoking bans and alcohol taxation. Added value of this study This review uses natural experiments among migrants to contribute to the existing evidence base by synthesising insights on the causal mechanism of contextual effects. It uses migration as an example to assess how contextual factors, ranging from policy environments to neighbourhood characteristics, generate or exacerbate inequalities among societies. We thereby circumvent and avoid limitations of other reviews on these topics, by exploiting five main sources of variation of contextual exposures: residential dispersal, arbitrary eligibility cut-offs, on-/off-timing of events, regional variation, and place of birth. Based on these, we identify three main types of natural experiments among migrant populations: 1) Studies “using” migration as an example to analyse contextual health effects or neighbourhoods in the post-migration phase; 2) Studies examining interactions between changes in environmental factors following migration processes as compared to those staying behind; and 3) Studies using natural experiments to study policy effects. The synthesised evidence confirms and provides causal evidence for the negative effects of neighbourhood disadvantage on physical health and mortality, while effects on mental health are mixed. The body of literature demonstrates that migration processes can unfold detrimental effects on physical health and mortality through adverse post-migratory contexts, while also demonstrating favourable effects for mental health and child health depending on the respective context. Our synthesis further provides causal evidence for the negative impacts of restrictive migration and social policies on healthcare utilization, mental health and mortality as well as the positive health effects when restrictions are lifted. Implications of all available evidence The evidence presented here demonstrates the health disadvantages faced by migrants in the immediate post-settlement phase, which are exacerbated by restrictive health, social and visa policies. More broadly, however, the evidence points to neighbourhood disadvantage as a crucial and causal mechanism underlying health inequities at a societal level. At the same time, studies uncover the potential of health and welfare programs to counteract the disadvantages created by othering processes and instead promote healthy contexts. Such evidence is valid beyond migrant populations and allows inference of the positive effects of inclusive health and welfare programs for other marginalized groups and the population as a whole. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study was funded the German Science Foundation (DFG) in the scope of the NEXUS project, a part of the PH-LENS research consortium (FOR 2928 / GZ: BO 5233/1-1). ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes A full list of studies identified in the search as well as the full data extracted from included studies are available for academic research projects by request to the corresponding author: Kayvan.bozorgmehr@uni-bielefeld.de