Professor James Potash explains that twin studies dating back to the 1920s have identified bipolar disorder as a genetic disorder.
So there have been studies going back to the 1920s demonstrating that manic depressive insanity, as it was originally called, has a genetic basis. There were studies in the 1920s in Germany that showed that manic depression runs in families, and also showing that identical twins are more likely to share both having the illness than are fraternal twins. That suggests the genetic basis for the illness since identical twins share 100 percent of their DNA and fraternal twins only 50 percent. In the modern era there have been many more studies of that sort – family studies showing that bipolar disorder and depression run in families and twin studies showing that increased sharing or concordance as we call it in identical twins as compared to fraternal. There have also been a couple of adoption studies showing that the biological parents of adoptees with the mood disorder are more likely to have the illness than are the adoptive parents. So all of those sorts of studies have very firmly established that there is a genetic basis to these illnesses, now we are at the point of trying to figure out what exactly are the genetic variations.