Risk for schizophrenia in intercountry adoptees: a Danish population-based cohort study

Background: Increasing numbers of intercountry adoptees are reaching adulthood, the age of onset for most serious mental disorders. Little is known about the development of schizophrenia in intercountry adoptees, a group with potentially increased vulnerability. The aim of this study was to investigate the risk of developing schizophrenia in adoptees and in non-adoptees. Methods: Utilising data from the Danish Civil Registration System, we established a population-based cohort of 1.06 million persons resident in Denmark before the age of 15, whose legal mother lived in Denmark at the child's birth. Intercountry adoptees were identified as children born abroad. Record linkage provided information on psychiatric admissions. Results: Intercountry adoptees had an increased relative risk (RR) (RR = 2.90, 95% CI 2.41-3.50) of developing schizophrenia compared to native Danes. The increased risk was independent of age at onset and age at, or region of, adoption, and was not attributable to mental illness in a foster parent, the foster parent's age, or to urbanisation. The foster mother's own biological offspring had also an increased risk of developing schizophrenia (1.92, 95% Cl 1.23-3.02). Conclusions: Young adult intercountry adoptees are at increased risk for schizophrenia. Although the underlying cause is unknown, a complex interplay of factors presumably may be involved, including heredity, adversity prior to adoption, and post-adoption adjustment difficulties during upbringing.