Feeling fear is an evolutionary survival tactic. A small number of people have a rare condition that means they're not scared of anything. How do they live a life without fear?

Imagine jumping out of an aeroplane and feeling nothing. No rush of adrenaline, or quickening heartbeat.

That is the reality for Jordy Cernik, a British man who had his adrenal glands removed to reduce anxiety caused by Cushing's syndrome – a rare disease which occurs when the adrenal glands produce too much cortisol, a stress hormone.

The treatment worked a little too well. Jordy stopped feeling anxious – but something was wrong. On a 2012 trip to Disneyland, he went on a rollercoaster ride and realised that he felt no fear. He subsequently skydived out of a plane, zip-wired off the Tyne Bridge in Newcastle and abseiled down the Shard in London – all without feeling the smallest raised pulse.

Cernik's experience is rare, but not unique. It may sound familiar to anyone who lives with Urbach-Wiethe disease (also known as lipoid proteinosis), a genetic condition so rare that only about 400 people have ever been diagnosed with it.

One famous Urbach-Wieth patient, known as SM, has been the subject of scientific studies at the University of Iowa in the US since the mid 1980s. In the early 2000s, Justin Feinstein was a graduate student when he joined the team, and began looking for ways to frighten SM.

"We showed her every single horror movie we could find," says Feinstein, now a clinical neuropsychologist at the Float Research Collective, which promotes Floatation-Reduced Environmental Stimulation Therapy (Rest) as a treatment for pain, stress, anxiety and related conditions.

Yet neither the Blair Witch Project, Arachnophobia, The Shining, or Silence of the Lambs elicited any fear in her. Even a tour of Waverley Hills Sanatorium, a creepy haunted house, had no effect.

"We exposed her to real life threats like snakes and spiders. But not only did she show a pronounced lack of fear, she couldn't help but approach them," says Feinstein. "She had this almost overwhelming curiosity to want to touch and interact with the different creatures."

Urbach-Wiethe disease is caused by a single mutation in the ECM1 gene, found on chromosome 1. ECM1 is one of many proteins crucial for maintaining the extracellular matrix (ECM), a supportive network that holds cells and tissues in place. When ECM1 is damaged, calcium and collagen begin to build up, causing cell death. One part of the body that seems to be particularly vulnerable to this process is the amygdala, an almond-shaped region of the brain long thought to play a role in processing fear.

In SM's case, she stopped feeling fear when Urbach-Wiethe disease destroyed her amygdala.

"What's remarkable is that it is specific to fear – her ability to process other types of emotion are mostly intact, whether that be happiness or anger or sadness," says Feinstein.

However, the story is actually more complicated than this. It turns out that the amygdala may play more of a role in certain types of fear than others. For example, it seems to be crucial for

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