Persons within the study base were linked via their personal identifier to the Danish Psychiatric Central Research Register27 to obtain information on mental disorders. The Danish Psychiatric Central Research Register was computerized in 1969 and contains data on all admissions to Danish psychiatric in-patient facilities. Information on outpatient visits was included from 1995 onwards. From 1994 onwards, the International Classification of Diseases, 10th revision, Diagnostic Criteria for Research was used for diagnostic classification.28 All persons within the study base, who had a diagnosis of schizophrenia, bipolar disorder, affective disorder, autism and attention-deficit/hyperactivity disorder were included (Table 1). At the time of linkage, the Danish Psychiatric Central Research Register contained all psychiatric contacts until 31 December 2012. Table 1 summarizes the number of individuals across the diagnostic groups.
