critically examining population-based studies
Early studies suggested higher rates of psychiatric morbidity among people who migrated.51;52 There are several possible explanations: ®rst, those with mental illness or a propensity to developing mental illness may selectively migrate. This could be facilitated if immigration is easy; the higher rate of mental hospital admission in England and Wales for patients from Scotland and Ireland has been cited as an example of this.53 But if barriers to immigration are strong then there could be a ``healthy migrant selection effect'' producing lower prevalence of mental illness in migrants.54 Second, the process and reasons for migration may in¯uence the development of mental illness. Migration may be enforced (for political reasons or natural disaster) or voluntary (for economic, career, or family reasons), and migrants may or may not remain segregated or adopt the host culture and life-style to varying degrees. Third, migrants may be subject to a variety of stresses associated with migration. These stresses include adaptation to a new environment, culture, society, and language, reduced support from family networks, and experience of racism and discrimination.27;54 Fourth, there may genuinely be a different prevalence of mental illness in the migrant's community of origin.