A family history of schizophrenia is the strongest single indicator of individual schizophrenia risk. Bipolar affective disorder and schizoaffective disorders have been found to occur more frequently in the parents and siblings of schizophrenia patients but it is less clear what the relationship is between schizophrenia and other mental illnesses in relatives. Researchers from the University of Aarhus in Denmark studied the records of everyone born in the country between 1955 and 1991 of whom 9,324 people developed schizophrenia. Schizophrenia was found to be strongly associated with schizophrenia and related disorders in first-degree relatives (siblings or parents) but almost any other mental disorder in people's close family increased their risk of developing the condition. Although the influence of relatives' schizophrenia was stronger the influence of other mental illnesses was greater overall because they are more common. The risk of developing schizophrenia was found to be 2.16x greater for people born in Copenhagen than for people born in rural areas but this risk dropped to 1.8x when familial mental illness was taken into account suggesting that this is something which needs to be allowed for when people are studying the links between urbanization and schizophrenia.
Mortensen, P.B., Pedersen, M.G. and Pedersen, C.B. - Psychiatric family history and schizophrenia risk in Denmark: which mental disorders are relevant? Psychological Medicine February 2010, 40(2), 201-210