Research published in Current Biology showed the woman felt no fear in a variety of scary situations.
These included exposure to snakes and spiders, horror films and a "haunted house".
The woman feels other emotions but said as an adult, she had never felt afraid.
She is the first known case of someone who is unable to process fear.
Researchers at the University of Iowa said her inability to feel frightened was because she is missing a structure in her brain called the amygdala.
The structure has long been associated with emotional learning - experiments in animals have shown that removing it makes them fearless.
However, it has never been observed in a human before. FULL STORY