Fear of snakes: An evolutionary perspective on the way kids learn

© 2011 Gwen Dewar, Ph.D., all rights reserved

Know a child with a disgust or fear of snakes?

How about a fearful toddler? Or baby?

People aren’t born with such responses. We know that from experiments on infants. If you show snakes to 7-month old babies, they don’t act frightened at all.

How—and when—do these fears first appear?

According to the idea of classical conditioning, people and other animals become frightened of a thing if they’ve experienced something nasty (like a shock) each time they encounter it.

But this process seems inefficient. Does a monkey need to get attacked in order to learn a fear of snakes?

If so, how many monkeys would survive to adulthood?

So the reality is this: Animals have ways of learning about dangerous things that don’t depend on first-hand experience. They can learn about predators by watching other members of their own species.

When a group of monkeys encounter a snake, the babies and juveniles watch how the adults react. They learn to cry out, to alert other members of their family, to get out of the snake’s reach. They learn to be afraid.

That’s fascinating enough. Humans aren’t the only creatures that pass knowledge from parents to offspring.

But there’s even more going on, and it concerns the way human children learn about danger:

Our brains might come equipped with special mechanisms that help us learn more quickly about certain kinds of animals—the animals that have posed the greatest threats to our ancestors.

Forty years ago, Martin Seligman proposed the idea that animals are “prepared” to learn some lessons very fast.

One example concerns potentially poisonous foods. Ever notice what happens if you happen to feel ill (and vomit) after eating a new food? You don’t want to try it again. For some people, it takes only one bad experience to learn this lesson—whether or not the food really is to blame.

Maybe snakes and spiders are like that. Maybe it takes very little to trigger our fear or disgust. We see friends or family act fearfully, and we’re persuaded. We don’t need a lot of time for the lesson to sink in. Our brains see a snake, receives the social input and says “Oh yes, your friends are right--those things are BAD.”

Where’s the evidence? Fast forward to the 1980s, when Michael Cook and Susan Mineka (1989) conducted classic experiments on captive rhesus monkeys.

These primates had never been in the wild, and they’d never before seen a snake. If you showed these monkeys a toy snake, they didn’t react fearfully.

So the researchers tried this. They divided the monkeys into two groups, and showed each group a different “spooky” video:

• Group One watched a video of a monkey acting frightened of a plastic flower.

• Group Two watched a video of a monkey acting frightened of a plastic snake.

The videos had been carefully edited to make it appear that the protagonist was equally fearful of both objects. But the videos were not equally effective.

If, after watching these videos, you gave a monkey a plastic flower, he wasn’t likely to react. But if you gave him a plastic snake, he showed fear.

In a similar experiment, monkeys rapidly learned to fear a toy crocodile but not a toy rabbit.

The results were exciting. The monkeys had developed a fear of snakes(and crocodiles) after a few, brief experiences watching another monkey on TV. It wasn’t a general response to social cues because the monkeys seemed relatively resistant to “catching” a fear of flowers or rabbits.

And it made sense that primates might have evolved specialized brain mechanisms for learning to fear snakes and crocodiles.

Snakes and crocs kill primates, and have done so for millions of years. So spotting these predators was a high-stakes game. And when the stakes are high enough, individuals who are quick to trust social cues about predators have a fitness advantage (Dewar 2003).

But what about human primates? Is there any evidence that human children are “prepared” by natural selection to learn that snakes are dangerous?

Detecting snakes in the grass

Child psychologists Judy DeLoache and Vanessa LoBue have found that American preschoolers are good “snake detectors.”

If you show three-year-olds a set of eight photographs—seven depicting caterpillars and one showing a snake—they are pretty quick to find the snake. By contrast, they take longer to find the caterpillar in a group of snake photos. The same thing happens when you ask kids to distinguish snakes and frogs. Picking out snakes seems to be easier (LoBue and DeLoache 2008).

DeLoache and LoBue have also tested the way babies—some as young as 7 months old--react to snakes and the sound of human fear (Deloache and LoBue 2009).

In one experiment, the researchers established that babies don’t respond fearfully to the sight of snakes. Not if the snakes are on video and the babies aren’t given any social hints that snakes are scary.

Next, the researchers asked a different question: Do babies respond differently to snakes if they hear adults sounding fearful?

To find out, LoBue and DeLoache presented 48 infants with a special “snake show.”

Each baby sat with his or her mother while two silent videos—running side by side—played simultaneously. One video showed an undulating snake. The other video depicted a non-snake moving at approximately the same speed. The moms were blindfolded so they couldn’t give their babies any cues.

Babies watched videos for a total of 12 trials—each trial pairing a different snake video with a video of a different non-snake (giraffe,rhinoceros, polar bear, hippopotamus, elephant, and large bird).

And here’s the important part. In half the trials, the videos were accompanied by an auditory track of an adult speaking in frightened tones. In the remaining trials, the videos were paired with a happy adult voice.

When given the choice, which videos did the babies watch?

It depended on the context.

When the videos were accompanied by the sound of an adult's fearful voice, the babies spent more time looking at the snake video.

When the videos were matched with the sounds of a happy voice, babies did not pay any special attention to the snake.

The mounting evidence for evolutionary biases

Have scientists demonstrated that there are specialized “snake-detectors” in the brain? Can we conclude that humans are “hard-wired” for speedy learning about snakes?

Not yet. We have to consider the possibility that these children had already learned something about snakes (or the other animals) before they participated in the experiments.

And even if we assume that the kids were “snake naïve,” it’s not yet clear how specifically snake-like an object must be to trigger these effects.

But DeLoache and LoBue have narrowed the possibilities. In a follow-up to the video test, they ran a similar experiment using still photographs. This time, babies didn’t pay any special attention to snakes—regardless of what sorts of voices they heard.

So LoBue and Deloache suspect it’s the distinctive writhing movement of snakes that really sets people off.

Meanwhile, I think the “prepared learning” hypothesis deserves our serious attention.

In a related line of research, anthropologist Lynn Isbell has argued that snakes have driven the evolution of primate 3-D, color vision—the better to detect serpentine predators (Isbell 2006).

And I’m intrigued by the results of another “snake detection” experiment—one like the “find the snake among the caterpillars” study, but with an interesting twist.

In this experiment, Nobuo Masataka and colleagues (2010) asked people to pick out a snake image from an array of flower images. And the twist? In some trials, the snake was at rest. In others, the snake was in “attack posture,” coiled and prepared to strike.

What Masataka’s team found was that people were a bit faster identifying snakes when the snakes had adopted an attack posture.

And the results seem especially compelling for two reasons:

(1) The visual difference between the “resting” and “attack posture” images was actually quite subtle (see the illustration for examples).

(2) The effect was found in both adults AND young children (aged 3-4).

According to their parents, these young children had never been exposed to snakes before. Not only had they never seen a real snake, they’d never seen any images of snakes. Or toy snakes.

If results like that can be replicated elsewhere, that’s a pretty impressive finding. Naïve kids spot snakes faster when the snakes are ready to strike? That’s just the sort of thing we’d like a predator-detection system to do for us.

And, in case you are wondering , there is evidence that a distinctively fearful reaction might help people detect snakes. In studies that compared snake-phobic adults with their non-phobic counterparts, the snake-fearing people were quicker at detection (Peira et al 2010; Ohman et al 2001).

That wasn’t true for the preschoolers. Kids with snake-fear were not any faster at detection. So perhaps the detection advantage develops over time.

Want to read more about predator detection and the fear of snakes?

An overview of the field

Authors Vanessa LoBue, David H. Rakison and Judy S. DeLoache have written a concise and up-to-date review of the research on biases for detecting creepy crawlies in children and infants:

(2010) Threat Perception Across the Life Span : Evidence for Multiple Converging Pathways. Current Directions in Psychological Science 19(6) 375-379.

You can also visit this page featuring many publications from DeLoache's lab, including downloadable papers about the evolutionary basis for a fear of snakes.

Thoughts about the evolution of sex differences

Fear of snakes and spiders is more common among women. For example, in a Swedish survey, snake or spider phobias were reported by about 12% of women but only 3% of men (Frederickson et al 1996).

Why the difference? Some researchers speculate that ancestral females were under greater selective pressure to avoid snakes and spiders--either because they encountered them more often (during foraging) or because they had to be extra-vigilant in order to protect the young children in their care.

Is there support for the idea that females have a stronger evolutionary bias for responding to snakes? DeLoache and LoBue haven’t found any sex differences in snake-detection abilities of young children.

But other research suggests that baby girls might be faster at learning to associate snakes and spiders with fearful faces (Rackison 2009).

I discuss this research--and question the reliability of surveys about the fear of snakes--in this article "Is snake fear a girly thing?"

You can also read David Rackison’s scientific paper, "Does women's greater fear of snakes and spiders originate in infancy?"



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References: Fear of snakes

Cook M and Mineka S. 1989. Observational condition of fear to fear-relevant versus fear-irrelevant stimuli in rhesus monkeys. Journal of Abnormal Psychology 98 (4): 448-459.

DeLoache J and LoBue V. 2009. The narrow fellow in the grass: Human infants associate snakes and fear. Developmental Science 12: 201–207.

Dewar G. 2002. The cue reliability approach to social transmission: designing tests for adaptive traditions. In: DM Fragaszy and S. Perry (eds), The biology of traditions: Models and Evidence. Cambridge University Press.

Fredrikson M, Annas P, Fischer H, and Wik G. 2001. Gender and age differences in the prevalence of specific fears and phobias. Behav Res Ther. 34(1):33-9.

Isbell, L.A. 2006. Snakes as agents of evolutionary change in primate brains. Journal of Human Evolution 51:1-35

LoBue V and DeLoache JS. 2008. Detecting the snake in the grass: Attention to fear-relevant stimuli by adults and young children.Psychological Science, 19, 284–289.

Masataka N, Hayakawa S, and Kawai N. 2010. Human young children as well as adults demonstrate 'superior' rapid snake detection when typical striking posture is displayed by the snake. PLoS One. 5(11):e15122.

Peira N, Golkar A, Larsson M, Wiens S. 2010. What you fear will appear: detection of schematic spiders in spider fear. Exp Psychol. 57(6):470-5.

Rackison D. 2009. Does women's greater fear of snakes and spiders originate in infancy? Evolution and Behavior. 30(6): 439–444.

Seligman MEP. 1970. On the generality of the laws of learning. Psychological Review 77(5): 406-418.

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