Many people squirm at the thought of a spider dangling above them or a snake slithering underfoot. In fact, research shows that at least five percent of the population has a strong, inhibiting fear of spiders and snakes.

But do we learn this fear, or is it something we're born with?

A group of researchers from the Max Planck Institute in Germany and Uppsala University in Sweden decided to find out by testing a segment of the population least likely to show fear: babies.

Forty-eight six-month-old infants were tested at the institute to analyze how they reacted to images the researchers predicted might be frightening. While sitting on their parents' laps, infants were shown images of spiders and snakes on white backgrounds for five seconds. To prevent parents from inadvertently influencing their infants' reactions, they were given opaque sunglasses during the experiment that prevented them from viewing whatever image was shown.

When the babies saw pictures of the snakes and spiders, they consistently reacted with larger pupils than when they were shown control images of flowers and fish. This finding, published in the journal Frontiers in Psychology, suggested that a fear of these creatures could be innate.

That's because dilated pupils are associated with activity in the noradrenergic system in the brain, the same system that processes stress. Closely measuring changes in pupil size has been used in previous studies to determine a variety of mental and emotional stress in adults.

"There was a definite stress response in the brain," said lead researcher Stefanie Hoehl. She noted that it's difficult to characterize the exact nature of the type of stress infants experienced, but dilated pupils show heightened states of arousal and mental processing. Rather than indicating fear in particular, the study says this shows an intense focus.

"The current work, and indeed no existing work, has provided evidence that fear of snakes or spiders is innate," said David Rakison, a psychology professor at Carnegie Mellon University who researches early infant development.

"Infants possess a specialized fear mechanism that means that they are 'prepared' to learn quickly that snakes and spiders are associated with a specific emotional or behavioral response," he noted.

To explain this innate focus, Hoehl points to a human evolution that has coincided with historically dangerous snakes and spiders.

A History of Fearing Snakes and Spiders

"It's a very long period of coevolution—nearly 40 to 60 million years of it, that early human ancestors and spiders and snakes have interacted," Hoehl explained. A venomous bite from one of these creatures lurking hidden in the grass could have left early human ancestors incapacitated or dead. Therefore, Hoehl's study claimed, humans' innate fear of these animals could serve as a defense mechanism..

This claim is supported by previous studies in adults and children that have claimed to indicate an innate evolutionary fear of spiders or snakes.

In 2001, a study published in the Journal of Experimental Psychology asked a group of university students to identify photos that depicted sources of fear. Consistently, students identified snakes and spiders as more dangerous than other photos of mammals or fungi.

"Snakes have provided a recurrent threat throughout mammalian evolution. Individuals who have been good at identifying and recruiting defense responses to snakes have left more offspring than individuals with less efficient defense systems," the study's coauthor, Arne Öhman, told National Geographic at the time.

A study published in 2008 in the journal Cognition, and another in 2014 in the journal Evolution and Human Behavior, also point to an inherited fear of spiders and snakes.

Snake Lovers and Spider Enthusiasts

If we're born with an innate feeling of stress toward spiders and snakes, that doesn't account for why some people grow up to have a crippling fear of these creatures while others keep them as pets.

Not all studies have concluded that fear of spiders and snakes is innate. A paper published in the journal Current Directions in Psychological Science found that seven-month-old infants noticed images of snakes more quickly but didn't show signs of fear. This indicated children may not have innate fears of these creatures but could identify them more readily.

Social learning partly accounts for this discrepancy, said Hoehl. Infants in the study were specifically capped around six months of age before they learned to crawl and walk. As babies become more mobile, they begin to learn about their natural surroundings. Research performed at New York University's Infant Action Lab in 2015 found that infants begin to learn how to tackle heights as they become more familiar with their environments and develop better depth perception.

Parental reinforcement plays a big part in how much fear grows, said Hoehl. While a baby bitten by a snake or spider might develop a strong association between the animals and danger, how a parent reacts to these creatures would also influence their child.