Increased incidence rates of schizophrenia in immigrants still lack a satisfactory explanation. The aim of this study was to examine the hypothesis that risk factors for schizophrenia also increase the risk of emigration to a foreign country. If valid, Danes emigrating from Denmark carry a higher predisposition to develop schizophrenia compared to Danes living in Denmark.
Utilizing data from the Danish Civil Registration System, we established a population-based cohort of 1.10 million native Danes. We assessed relative risks of emigration to a foreign country in relation to sex, age, urban birth, parental age, and a history of mental illness.
Urban birth in Denmark was a significant predictor of emigration to a foreign country. A maternal history of psychiatric contact during childhood and a parental history of bipolar affective disorder increased the risks of emigration. A personal history of mental illness decreased the risk of emigration, mostly for people diagnosed with schizophrenia.
Our study provided evidence that Danish emigrants residing in a foreign country have both a higher predisposition of schizophrenia due to differential exposure to birth in urban areas and a lower predisposition of schizophrenia due to differential exposure to a history of mental illness. Although competing selection mechanisms operate, the combined effect of these different selection mechanisms was limited, thus suggesting a potential role for yet to be identified adverse environmental effects operating either before or after emigration.