Science Puts Fear in Fearless Person's Heart

Fearless? Not if science can help it! Science can scare anybody, even those who are born with a rare genetic disorder that make them fearless.
But first, let's talk about the amygdala, a small almond-shaped structure deep inside the brain that's been dubbed "the seat of fear." Patients with Urbach-Wiethe disease have atrophied amygdalas, and as a result, they experience no fear.
Justin Feinstein of the University of Iowa and colleagues posited that because these patients don't have the necessary brain structure for fear, they'd be immune to things that would scare a normal person:

One situation in which the amygdala triggers fear and panic attacks is when it detects unusually high concentrations of carbon dioxide — a sign of possible suffocation — by sensing increased acidity in the blood. This may occur even if CO2 is inhaled in concentrations that are not lethal. Feinstein and his colleagues therefore predicted that patients with damaged amygdalas would not feel fear after inhaling the gas.
To test this, they asked S.M. [a patient], two other patients with Urbach-Wiethe disease, and 12 healthy controls to inhale 35% carbon dioxide through a mask. To their surprise, the researchers found that the brain-damaged patients did experience fear immediately after inhalation — and, in fact, became even more fearful and panicky than did the healthy volunteers.
“The patients experienced significantly more fear and panic than the controls,” says Feinstein. In interviews conducted afterwards, all three patients reported feeling scared of suffocating and dying while inhaling the gas. For S.M., this was the first time she had experienced fear since childhood.

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