Genetic factors appear to play a more significant role in the development of bipolar disorder than in major depressive disorder. This conclusion is drawn from two key observations: the risk of developing the disorder is substantially higher for relatives of individuals with bipolar disorder (over 9x) compared to those with major depressive disorder (2x), and the disparity in concordance rates between identical and fraternal twins is much greater for bipolar disorder (67% vs. 16%) than for major depressive disorder (50% vs. 38%).