Some people don’t like spiders and snakes, though they’re not sure why. It turns out you don’t actually need a reason to hate those creatures: According to German and Swedish researchers, the fear of spiders and snakes is so innate that even babies become anxious at the sight of them.

This, they say, was a simple matter of survival: For thousands of years, humans have been conditioned to avoid creepy crawlies—even before the humans themselves were able to crawl.

A study published recently in Frontiers in Psychology found images of snakes and spiders induced a stress response in 6-month-old infants, indicating these phobias are hereditary rather than learned. For the study, researchers showed the babies photos of spiders and snakes next to flowers and fish and noticed their pupils dilated when looking at the creepy crawlers, but not the non-threatening objects.

"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 lead researcher Stefanie Hoehl, neuroscientist at the Max Planck Institute for Human Cognitive and Brain Sciences and the University of Vienna. She explains this signals activations of the noradrenergic system in the brain, which is responsible for stress reactions.

Hoehl continues, “Accordingly, even the youngest babies seem to be stressed by these groups of animals."

The fear of spiders and snakes is among the most common phobias in the United States. According to the American Psychiatric Association, more than 10 million people live with some sort of phobia, and up to 40% of those fears involve bugs, mice, snakes and bats.

For decades, scientists have debated whether or not we learn to fear animals, objects or activities or if those fears are evolutionary in origin. These new findings suggest that some phobias, such as the fear of spiders (arachnophobia) or snakes (ophidiophobia), are intrinsic.

As for the why, Hoehl has a theory: "We assume that the reason for this particular reaction upon seeing spiders and snakes is due to the coexistence of these potentially dangerous animals with humans and their ancestors for more than 40 to 60 million years and therefore much longer than with today's dangerous mammals.”

Of course, that’s not true about other risky objects. As all parents know, when it comes to knives or light sockets, babies love to run toward danger. But, as the study suggests, give humans a few million years and the babies of tomorrow may indeed know better.