PARIS-Many people would shiver at the thought of a spider crawling along their arm, or a snake slithering up their leg.

But in western, industrialized countries, especially in middle Europe, most people have never come across a poisonous spider or snake in the wild.

Now, researchers may know where this fear comes from - we’re born with it.

Even babies fear images of spiders and snakes, and researchers say the reaction could have been embedded in the brain for an evolutionarily long time due to the coexistence of these potentially dangerous animals with humans and their ancestors for more than 40 to 60 million years. The fear of spiders, arachnophobia, can develop into anxiety which limits a person’s daily life.

Some people can’t even enter a room unless it’s been declared ‘spider free,’ if the fear is severe.

In developed countries, one to fiver per cent of the population are affected by a real phobia of spiders or snakes. 

Until now, researchers with the Max Planck Institute for Human Cognitive and Brain Sciences (MPI CBS) say that it wasn’t clear where this widespread anxiety stems from.  While some researchers assume that we learn this fear from our surroundings when we’re children, other say that it’s innate.

However, previous studies on this topic are flawed in that they were conducted with adults or older children - making it hard to determine which behaviors were learned and which were innate.

These sorts of studies with children only testes whether they spot spiders and snakes faster than harmless animals or objects, not whether they show a direct physiological fear reaction.

So researchers at MPI CBS in Leipzig, Germany and the Uppsala University in Sweden conducted a study which found that even in infants, a stress reaction happens when they see a spider or snake.

They found that this happens in infants as young as six months-old, when they are still very immobile and have not had much opportunity to learn that these animals can be dangerous.

‘When we showed pictures of a snake or a spider to the babies instead of a flower or a fish of the same size and color, they reacted with significantly bigger pupils,’ says Stefanie Hoehl, lead investigator of the underlying study and neuroscientist at MPI CBS and the University of Vienna.

‘In constant light conditions this change in size of the pupils is an important signal for the activation of the noradrenergic system in the brain, which is responsible for stress reactions.

‘Accordingly, even the youngest babies seem to be stressed by these groups of animals.’

The researchers concluded that the fear of snakes and spiders is of evolutionary origin, and similarly to primates or snakes, mechanisms in our brains allow us to identify objects and to react to them very quickly.