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Growing up in the 1970s, Jary Larsen heard his mother say their extended family had been haunted for years by a strange disorder. It changed personalities and caused odd behavior. She called it Larsen disease.

Larsen, the ninth of 10 children raised on a farm in Minnesota, was always skeptical — until 2006, when his older brother Peter drove a car into a ditch. Police found him sitting listlessly behind the wheel, confused about which direction he had been heading.

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