Risk of schizophrenia in second-generation immigrants: a Danish population-based cohort study

Abstract

Background. Urban birth, a risk factor for schizophrenia, is more frequent among second-generation immigrants. The aim of the current study was to determine whether the increased risk for schizophrenia found in second-generation immigrants is explained by the degree of urbanization of birthplace and/or factors related to parentage, such as geographic origin or history of residence abroad during upbringing.

Method. Using data from the Danish Civil Registration System (CRS), we established a population-based cohort of 2·0 million Danes (persons born in Denmark). Schizophrenia in cohort members was identified by cross-linkage with the Danish Psychiatric Central Register.

Results. The relative risk of developing schizophrenia was 1·93 [95% confidence interval (CI) 1·79–2·08] and 2·96 (95% CI 2·49–3·51) among persons with one or both parents foreign-born respectively compared to native Danes. Adjustment for urbanization of birthplace and parental characteristics reduced these risks slightly. However, urbanization had a lesser effect in second-generation immigrants than in Danes. History of residence abroad was a risk factor for schizophrenia, regardless of whether parents were foreign-born or native Danes.

Conclusion. The increased risk found in second-generation immigrants cannot be explained by urbanization or parental characteristics pertaining to age, mental illness, geographic origin or residence abroad during a child's upbringing.