Urbanicity during upbringing and bipolar affective disorders in Denmark

Background: It has been suggested that known or suspected risk factors for schizophrenia may also be of importance for other psychoses, but the empirical evidence regarding this is limited. Urbanicity of place of birth and during upbringing has been shown to be related to the risk of schizophrenia. Few studies of urbanicity in relation to bipolar affective disorder exist. Objective: To investigate the potential association between urbanicity at birth and during upbringing and the risk of bipolar affective disorder. Method: Using data from the Danish Civil Registration System, we established a population-based cohort of 2.04 million people born in Denmark during 1956-1986, which included information on place of residence during upbringing. Bipolar affective disorder in cohort members was identified by linkage with the Danish Psychiatric Central Register. Results: Overall, 2232 people developed bipolar affective disorder during 1971-2001. We found evidence of an increased risk associated with residence in the provincial city; individuals, who at the 15th birthday lived in the provincial city, had a risk of 1.23 (1.08-1.41). This increased risk was explained solely by an increased risk associated with residence in Aarhus; at the 15th birthday, people residing in Aarhus - the largest of the three Danish provincial cities - had a 1.83 (1.56-2.14) increased risk of bipolar affective disorder (p < 0.001). Urbanicity during upbringing (p = 0.13) had no significant effects on the risk of bipolar affective disorder. Conclusions: We found no evidence of a dose-response relationship between urbanicity at birth (and during upbringing) and the risk of bipolar affective disorders in Denmark, but found some evidence that the diagnostic practices used in Aarhus differed from the rest of Denmark.