ABSTRACT
It has long been recognized that manic depression or bipolar disorder (BD) runs in families and therefore a familial and/or genetic component in the aetiology of BD has been postulated. Epidemiological research involving family, twin and adoption studies led to the observation that genetic factors do confer susceptibility to BD.1 The overall prevalence of BD appears to be about 1% in the general population. In contrast, first-degree relatives have a 5-10-fold increased risk of developing BD, while it appears that the risk for second-degree relatives falls between risks for first-degree relatives and the general population. Concordance for BD has consistently been found to be significantly higher in monozygotic than in dizygotic twins (about 50% versus 10%, which is similar to the rate for first-degree relatives).1,2 These findings point to some important conclusions: the clustering of BD in families is based on the presence of genetic factors; however, since penetrance is incomplete, the presence of specific non-genetic (epigenetic and/or environmental) factors is likely to influence the occurrence of BD.3 Finally, it is generally accepted, based on statistical models, that not one but many (or at least several) genes of moderate or small effect contribute to BD.4
