I am not a fan of the phrase "I told you so", but I've been saying this for years: schizophrenia and bipolar disorder (manic depression) do not only share heredity as a primary cause, with environmental factors secondary, but the two diseases are related.

These should, in fact, be considered two illneses on a psychotic-affective spectrum with many shared characteristics. Bipolar disorder, and major depression at that, can have psychotic features, hallucinations, delusions and such, and many people with schizophrenia have mood swings or at least depression. Antipsychotics, especially the newer atypicals (Risperdal, Clozaril, Zyprexa, Seroquel, Geodon, Ability, Invega etc.), were originally intended for schizophrenia-type disorderss but are now used to treat bipolar disorder. However, lithium and the anticonvulsants found to be effective for mania (Depakote/valproate, Tegretol, Lamictal and so on), if used for psychotic disorders, are only prescribed for the moods of schizoaffective disorder, which sits on the middle of the spectrum.

Also, take into account that SSRIs and other antidepressants may not be all that effective for bipolar disorder, suggesting that BD, despite its classification as a mood disorder, may not be that closely related to major depressive disorder.