A man in cowboy hat jumps the fence at the Boston Marathon to apply homemade tourniquets on severed limbs. A Vietnam vet jumps into the subway tracks to shield a man he has never met from an oncoming train. Are heroes born or made? "We really don't know for sure why people put their lives on the line for other people," said Frank Farley, a professor of psychology at Temple University. "It is the most mysterious act, in my view. How does someone get close to giving up their life for someone else they may not even know?" he asked.