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Fear free. SM held a snake (left), tried to touch a tarantula (center), and thoroughly enjoyed a haunted house (right).

Credit: J. Feinstein et al., Current Biology (2010)

Fear free. SM held a snake (left), tried to touch a tarantula (center), and thoroughly enjoyed a haunted house (right).

Credit: J. Feinstein et al., Current Biology (2010)

"SM" is a bit of an emotional anomaly. The 44-year-old mother, given those initials to preserve her anonymity, isn't scared of snakes. She doesn't shriek when she sees a scary movie. Even haunted houses don't give her chills. SM is pretty much fearless—and now scientists think they've figured out why.

The study's lead authors met SM, who has a rare genetic condition called Urbach-Wiethe disease, more than 2...