Background: A family history of schizophrenia is the strongest single indicator of individual schizophrenia risk. Bipolar affective disorder and schizo-affective disorders have been documented to occur more frequently in parents and siblings of schizophrenia patients, but the familial occurrence of the broader range of mental illnesses and their role as confounders have not been studied in large population-based samples.
Method: All people born in Denmark between 1955 and 1991 (1.74 million) were followed for the development of schizophrenia (9324 cases) during 28 million person-years at risk. Information of schizophrenia in cohort members and psychiatric history in parents and siblings was established through linkage with the Danish Psychiatric Central Register. Data were analysed using log-linear Poisson regression.
Results: Schizophrenia was, as expected, strongly associated with schizophrenia and related disorders among first-degree relatives. However, almost any other psychiatric disorder among first-degree relatives increased the individual's risk of schizophrenia. The population attributable risk associated with psychiatric family history in general was 27.1% whereas family histories including schizophrenia only accounted for 6.0%. The general psychiatric family history was a confounder of the association between schizophrenia and urbanization of place of birth.
Conclusions: Clinically diagnosed schizophrenia is associated with a much broader range of mental disorders in first-degree relatives than previously reported. This may suggest risk haplotypes shared across many disorders and/or shared environmental factors clustering in families. Failure to take the broad range of psychiatric family history into account may bias results of all risk-factor studies of schizophrenia.