". In this community of quiet-spoken, humble pacifists, such behavior "really stands out against the social landscape," observes Medical Sociologist Janice Egeland, who has spent more than 25 years among the Old Order Amish, as the group is formally known. When it does occur, the Amish often have an explanation: "Siss im blut," they say; the peculiar behavior is "in the blood."
http://www.time.com/time/magazine/article/0,9171,963725,00.html
This is a very interesting article and, for anyone wanting to know more, there is plenty of information available. I'd like to point out that schizophrenia is the next level up from manic depression.
My opinion only
It would be interesting to see a comparison, percentage wise, of the level of incidence in the Amish community versus us 'English', as they refer to us. I appreciate the link - thanks.