Most of us blame it on scary movies or a past trauma.But scientists now believe that fear, far from being a learned response, is actually programmed into our genes.
A study of more than 170 identical and non-identical twins by American and Swedish scientists has found fear is a partly-inherited trait.
The twins were shown various different images - some frightening ones such as snakes and spiders and others that were neutral, such as shapes - while being given a mild electric shock.
The scientists measured the twins' skin electrical conductivity, which normally increases during fear. The results showed identical twins - who have the same genes - had the same responses, but the non-identical twins - who are genetically different - had differing responses, said Archives of General Psychiatry.
The scientists said the study, which would help future treatment of phobias and anxiety disorders, revealed that between one-third and one-half of fear was controlled by genes and was not purely the result of conditioning.