Causes
The causes of bipolar disorder likely vary between individuals. Twin studies have been limited by relatively small sample sizes but have indicated a substantial genetic contribution, as well as environmental influence. For bipolar I, the (probandwise) concordance rates in modern studies have been consistently put at around 40% in monozygotic twins (same genes), compared to 0 to 10% in dizygotic twins. A combination of bipolar I, II and cyclothymia produced concordance rates of 42% vs 11%, with a relatively lower ratio for bipolar II that likely reflects heterogeneity. The overall heritability of the bipolar spectrum has been put at 0.71. There is overlap with unipolar depression and if this is also counted in the co-twin the concordance with bipolar disorder rises to 67% in monozigotic twins and 19% in dizigotic. The relatively low concordance between dizygotic twins brought up together suggests that shared family environmental effects are limited, although the ability to detect them has been limited by small sample sizes.
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