The long-running feud between the Hatfields and McCoys may have been due in part to a medical condition passed down in one of the families.
Dozens of McCoy descendants apparently have the disease, which causes high blood pressure, racing hearts, severe headaches and too much adrenaline and other "fight or flight" stress hormones.
No one blames the whole feud on this, but doctors say it could help explain some of the clan's notorious behavior.
Affected family members have long been known to be combative, even with their kin. [Rita] Reynolds recalled her grandfather, "Smallwood" McCoy.
"When he would come to visit, everyone would run and hide. They acted like they were scared to death of him. He had a really bad temper," she said. [Link]