File photo: Common phobias really are hardwired into our brains, research has revealed. Picture: Pat McKril

London - Babies are terrified of snakes and spiders by the age of six months – proving common phobias really are hardwired into our brains, research has revealed.

When the children were shown images of snakes and spiders, their pupils enlarged dramatically, signifying stress.

Infants cannot know reptiles and arachnids are potentially deadly – so the fear must be a protective mechanism present from birth, the study found.

Fear of snakes is one of the most common and intense phobias – with one in three adults worldwide suffering from it. A similar number of adults have arachnophobia.

But whether these are innate or learned has been debated for years. In the study, published in the journal Frontiers In Psychology, 32 Swedish infants took part in two experiments in which they were shown pictures of snakes and flowers, or spiders and fish.

Study author Dr Stefanie Hoehl, of the Max Planck Institute for Human Cognitive and Brain Sciences in Germany, said: "We conclude that fear of snakes and spiders is of evolutionary origin."