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There aren’t many people who don’t shiver at the thought of a spider creeping up their arm or a snake slithering underfoot. In fact, about 5 percent of the population has a strong, inhibiting fear of spiders and snakes. But where does this fear come from? After all, not many of us actually live anywhere near poisonous spiders or ever see a snake in real life. Do we learn it from watching others or listening to them talk about their fear? Or could it be something innate? A group of researchers from the Max Planck Institute in Germany and the Uppsala University in Sweden decided to find out.

The researchers tested 48 six-month-old infants to see how they reacted to images of these creatures. When the babies saw images of spiders and snakes, they reacted with larger pupils than when they were shown images of flowers and fish. Other studies have shown that dilated pupils are associated with stress, so, “There was a definite stress response in the brain,” said lead researcher Stefanie Hoehl about the babies’ reactions.

Because these babies were too young to have actually learned that snakes and spiders can be scary, the researchers concluded that the fear of snakes and spiders is innate, probably hard-wired into us as a defense mechanism, as a venomous bite from one of these creatures could kill or severely incapacitate our early human ancestors.

But why do some people keep these creatures as pets, others have a manageable fear and some people’s fear is debilitating? Researchers aren’t sure, but in future studies, Hoehl hopes to test how temperament influences spider and snake phobia.