Causes
The causes of bipolar disorder likely vary between individuals and the exact mechanism underlying the disorder remains unclear.[31] Genetic influences are believed to account for 60–80% of the risk of developing the disorder indicating a strong hereditary component.[14] The overall heritability of the bipolar spectrum has been estimated at 0.71.[32] Twin studies have been limited by relatively small sample sizes but have indicated a substantial genetic contribution, as well as environmental influence. For bipolar disorder type I, the (probandwise) concordance rates in modern studies have been consistently estimated at around 40% in identical twins (same genes), compared to about 5% in fraternal twins.[7][33] 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. 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 monozygotic twins and 19% in dizygotic.[34] 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.[32]