Why is depression the most prevalent presentation of the illness?
We don’t really know. What we do know is that the untreated, naturalistic course of mania differs from depression. Manic states tend to be somewhat self-limiting and briefer in duration, from several weeks to a few months. Hypomanic episodes tend to be even shorter in length, with some evidence citing a mean modal duration of only two days. So, these abnormal spikes in mood and/or energy tend to not only ignite abruptly but to burn so intensely and quickly as to burn themselves out. Often these manic or hypomanic episodes end with a rapid transition into depression. Depressions on average last longer than mania/hypomania, with the untreated course persisting for several months to more than a year. If we also then consider that we have fewer known effective treatments for bipolar depression than for bipolar mania, it should come as no surprise that the depressive experience is more prevalent for patients with bipolar disorder.
