Most of us would tremble when faced with a venomous spider or a dangerously high cliff edge.

But one woman has a rare disease meaning she is biologically unable to feel fear.

The U.S. woman, known to doctors as SM, has been put into life-threatening situations but remains completely unfazed.

She didn’t panic at being held at gunpoint and knifepoint – and didn’t even call the police after these attacks.

She was also nearly beaten to death by her first husband, but never felt scared.

A U.S. woman known to doctors as SM is biologically unable to feel the emotion fear, due to a genetic condition which has caused the part of her brain responsible for being scared to waste away

When faced with snakes that could be poisonous to her, she is so curious she has to be restrained from touching them.

While doctors have been studying SM for more than a decade, she has never been interviewed before.

This is because the team of neuroscientists, from the University of Iowa, looking into her condition, said if her identity was made public people could easily take advantage of her.

So Dr Daniel Tranel, of the University of Iowa, carried out the interview and passed it on to journalists at NPR radio station, to be used in their Invisibilia podcast.

Dr Tranel began by asking SM, a 44-year-old mother of three, to describe what fear is.

‘Well, that’s what I’m trying to – to be honest, I truly have no clue’ she replied.

She said she does remember being scared of a catfish her father had caught when she was a young girl, as she didn’t want it to bite her, but that was the only time she can remember experiencing the emotion.

And not being scared has led to some hair-raising situations.

WHAT IS URBACH-WIETHE DISEASE? Urbach–Wiethe disease is a rare genetic disorder. Scientists have only identified 400 cases since it's discovery. The symptoms of the disease vary greatly from individual to individual. They could include a hoarse voice, bumps around the eyes, easily damaged skin with wounds that don't heal easily. Other symptoms include dry, wrinkly skin, and a general thickening of the skin and mucous membranes. In some cases there is also a hardening of brain tissue in which can lead to epilepsy and other abnormalities, such as a lack of fear, in SM's case. The disease is typically not life-threatening and patients do not show a decreased life span. Advertisement

When her sons were small she was walking to the shop when a man on a park bench called her over.

She said: ‘He grabbed me by the shirt, and he held a knife to my throat and told me he was going to cut me.

'I told him - I said: “go ahead and cut me”. And I said, “I'll be coming back, and I'll hunt your ass”.

The man let her go, and she went home, but she didn’t call the police after this event, as she didn’t see the danger.

SM has a rare genetic condition called Urbach-Wiethe disease, which causes parts of her brain to harden and waste away, NPR reports.

Urbach-Wiethe disease has three main symptoms: people with the condition have a hoarse voice, small bumps around their eyes and calcium deposits in the brain.

These deposits cause parts of the brain to calcify and harden, which can lead to epilepsy or other abnormalities.

In SM’s case, almond-like structures called amygdala found deep in the brain have calcified and wasted away.

The amygdalae are crucial to the human fear response, and so SM is now literally unable to feel the emotion fear.

Normally, in situations that might cause danger, the amygdalae would send signals to the body that would induce symptoms of fear: the heart to start racing and the palms to sweat.

SM has Urbach-Wiethe disease, which causes calcium deposits to form in her brain. Deposits have formed in her brain's amygdalae (one of which is pictured in red), which are responsible for the fear response

But for SM, as this bit of her brain doesn’t exist, she doesn’t experience these symptoms in situations others would find scary.

She is also unable to recognise a fearful facial expression on another person.

Doctors note this is an isolated defect: she has normal intelligence and feels other emotions such as joy, sadness, anger in the same way as others.

Scientists have identified only about 400 people worldwide with Urbach-Wiethe disease.

University of Iowa scientists were able to induce fear in her in 2013, when they made her inhale carbon dioxide.

Even at low concentrations, if the amygdala detects carbon dioxide in the body, it normally triggers fear and panic as it is a sign of possible suffocation.

Scientists predicted that SM wouldn’t panic after inhaling the gas, but in fact, she did.

Professor Antonio Damasio, a neuroscientist at the University of Iowa who has been studying SM, says what is remarkable about her is that her lack of fear means she has no traumatic memories.

When faced with poisonous snakes that would make most people shiver with fright, SM is so curious she has to be held back from touching them. She has also been held at knifepoint, at gunpoint and nearly been beaten to death. Her genetic condition meant she didn't feel scared in these situations and has no bad memories

If bad events happen in SM’s life, they are not registered as bad or threatening, and she doesn’t experience them as such, he said.

Even if her husband almost beat her to death, the memory is not traumatic, and leaves no 'emotional footprint' which could cause psychological problems for others.

‘If you have no fear, more terrible things will happen to you, but you don't personally experience them as terrible,’ Professor Damasio told NPR journalists.

‘If you have a lot of fear, fewer bad things are likely to happen, but it's very probable that your life is more painful to you. So is it better to be fearful or fearless?’

Scientists are now looking into whether SM’s case could help others with traumatic memories, such as soldiers with Post Traumatic Stress Disorder, according to LiveScience.