You also have to look at the statistical anomaly of the increase of BP diagnosis in the last 20 years. If you look at the incidence of bipolar diagnosis over the last 40 years the incidence has increased dramatically. Along with the increase of diagnosis has also increased the number of medications out there used to treat bipolar disorder. I am not saying that everyone who is diagnosed "bipolar" is misdiagnosed/over-diagnosed, but having been diagnosed at age 13 with Bipolar in 1989, spent 15 years in and out of psychiatric hospitals 4-6 times a year and was 6 years ago on 11 different psych meds at the same time (I am no longer on any psych meds and haven't had a hospitalization in the last 6 years) but that took WORK, and lots of it.
My therapist uses BP as the "insurance DX" because it is consistent with the previous DX and he has no real way to explain why it is that I have a committee of 8 people in my head that I talk to and who comment on and have kinda equal decision making status in relation to that they have discussions (with me of course) about things. (I am not in any way other then the auditory and visual hallucinations symptomatic of Schizophrenic so that DX was quickly ruled out) They are there more often then they aren't and sometimes I see them manifest themselves as hallucinations who show up in my presence. Not specifically BP, but well I hadn't told them about "the committee" when I was 13 and in my first psych hospital.
The psychiatric field is relatively new compared to that of other scholastic studies. Freud, considered the father of Psychology only passed away in the 1940's,having created Psychology as we know it. There is much that they dont know about the brain, and they know even less about the mind. How is it that I am able to be a functioning normal (ok fairly normal) student who is successful in school as well as other ventures yet still have a collection of "friends" that she often has to be reminded by the SluttyUnicorn as well as DaJoshy that they are in the room and that I can talk to them instead. Often I still choose to talk to the committee but that is usually because we are talking about them.