Are humans hardwired to fear snakes and spiders? According to babies, yes

Story transcript

If you're afraid of spiders, the 1990 horror film Arachnophobia is probably not a favourite. And you probably also weren't a fan of the 1977 movie Kingdom of the Spiders, starring William Shatner as a tough-as-nails veterinarian.

But even if you saw those movies at an impressionable age, a new study suggests they didn't create that fear. Turns out our discomfort with hairy eight-legged creatures may be more hardwired.

• AS IT HAPPENS: Australian zookeeper asks people to catch this deadly spider for him

Researchers in Germany have discovered that a fear of spiders and snakes may have genetic links — and they did it by doing something a little bit, well, mean. 

Stefanie Hoehl is a professor of developmental psychology at the University of Vienna and the lead researcher on the study. She spoke to As It Happens host Carol Off about her findings.

Professor Hoehl, why did you want to scare babies?

Excellent question. We've known for some time that many people are actually afraid of spiders and snakes and there's really no good reason for it. Nowadays, in western societies, these animals rarely pose a threat to us.

Many people have wondered about this and, even in the '70s, there have been theories that actually there is an evolutionary background for these fears. However, the problem is, working with adults, we really cannot tell apart whether something is really innate, whether there is some evolutionary mechanism going on, or whether it is something that people have learned, or that is cultural. 

So babies become a better guinea pig for you?

Yeah, I wouldn't call them guinea pigs. But this is one of the reasons why it is really important and really interesting to work with infants because they haven't had so many learning experiences and cultural background doesn't play so much of a role there.

So the theory is that the fear of spiders or snakes or other things — other creepy, crawly things — is something learned. The parents react, your mother reacts, with fear and so the baby learns this. You wanted to find out if that was true or not or do they actually have some inherent fear of these things.

Yes, exactly. So what we found is a stress response to pictures of spiders and snakes in six-month-old infants. This is where we get a really good chance to look at mechanisms that are perhaps innate.

So how did you test it? Did you put spiders and snakes into their nurseries and see how they respond?

No, no, no. We tested them in a lab at Uppsala University in Sweden. In this lab we have an eye trigger. This is a machine that tells us exactly where the baby is looking on the screen and, at the same time, it tells us how big the pupil is — how much the pupil is dilated. This is a very interesting measure for us because it's a very good indicator of activation of the noradrenergic system in the brain, which is part of the stress response of the "fight or flight" response.

How do you know that they weren't just responding to any images that came up on the screen that fascinated them or maybe stressed them a bit?

It's really important to have very good control images that are matched in terms of colour, size and luminance because the pupil also reacts to light differences. For the spiders, we had flower images and, for the snakes, we had fish. So we compared infants pupil responses to these evolutionary-threatening images and compared these to the control images. 

And what did you find?

We found that infants responded, specifically to pictures of spiders and snakes, with increased pupil dilation. So this is a response that's not just in response to any kind of image and it's certainly not a response to luminance because this was perfectly matched.

Why do you think it's spiders and snakes? Kids love to look at pictures of lions, tigers, bears and other things that you think might create a fight or flight trigger in a human, if it is something they've inherited over time.

This is a very good question because it would make so much more sense to be inherently afraid of lions or tigers. However, what's really interesting about spiders and snakes is that they have been posing a threat to our ancestors for an immensely long time. Spiders and snakes developed venomous bites 40 to 60 million years ago.

This is a really long, long time of co-evolution and we think that this enables primates, not only humans, but other primates as well, to develop mechanisms that enable us to detect these animals very quickly, to respond to them, to put our bodies into fight or flight mode. This may really have posed an advantage. Nowadays, it really doesn't make so much sense. As I said, there are not so many animals, spiders, snakes that are dangerous to humans nowadays in most parts of the world where humans live.

This interview has been edited for length and clarity. For more on this story, listen to our full interview with Stefanie Hoehl.