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Migration is often considered a risk factor for mental disorders, because it entails changes in the social environment and, in some cases, the migration experience itself may be traumatic (1–3). However, evidence on the impact of migration on mental health remains mixed. Previous work has documented both positive (4, 5) and negative (6–12) associations between migration and mental health. These conflicting findings may partly reflect the multitude of migration experiences. For example, moving voluntarily to take up a new job likely affects mental health differently than having to flee an armed conflict. However, the mixed results may also arise from differences between studies in the extent and direction of selection bias. The key challenge in measuring the impact of migration is to evaluate what would have happened to the migrants had they not migrated. When people self-select into migration, nonmigrants are unlikely to constitute an appropriate control group that could be used to construct this counterfactual. Indeed, earlier work has found that people who choose to migrate have lower risk-aversion (13), higher cognitive ability, adaptability (14), and sociability (15), and more self-reported mental health problems (16) than non-migrants already before they migrate. Furthermore, the association between adverse outcomes and migration seems to attenuate when genetic and shared environmental confounds are controlled for (17). As ethical and practical considerations rule out randomized trials in the study of migration, researchers have started increasingly to study historical events such as armed conflicts (18–20) or administrative decisions …