Why Babies Fear Plants

The evolutionary science that might be behind this strange infant aversion
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There is a picture I have in my mind that captures an important facet of childhood. Its caption reads: "Small Human With Marble In Nose." We trot this image out at family gatherings—there is surely a relative on hand who was once such a Small Human—to illustrate the pleasant idea that children, at their most endearing and most exasperating, are explorers/inventors/geniuses in miniature. By this school of thought, jamming a small object up one’s nose is just a foray into The Scientific Method gone a bit awry.

But imagine this picture as a cartoon. Get rid of baby Einstein’s precocious smile, add in a runny nostril (the unobstructed one), and rewrite the caption: SURVIVAL OF THE FITTEST?

This is not just a joke. Developmental and evolutionary psychologists study babies very seriously because they believe that the not-quite-clean slates of baby brains can give us surprising insight into our evolutionary pasts. They reason that the early history of an individual human’s mind is a sort of analogy to the early history of all humankind. Because infant minds have not yet been shaped by the empirical world around them, their behavior is largely the product of elemental hard-wiring in the brain—the kind of circuitry that dates all the way back to our days as hunter-gatherers. In short, all the newfangled nurture of hip mommies and daddies has not yet taken hold in these minds, which leaves scientists with an unusually clear view of very old-fashioned nature.

A study published last week in the journal Cognition by a team of Yale researchers deployed this line of analysis and produced a headline that no doubt caught the eye of many Baby Bjorn-toting parents: Babies Reluctant to Grab Plants. The study reported that infant subjects took much longer to touch plants than a variety of other objects.

The study’s lead author, Annie Wertz, wants to be clear about what this research is not: “We are in no way claiming that infants can be left alone with plants or anything like that,” she says. She reels off this caveat dutifully because she wants to discuss what truly interests her about these latest findings. Wertz's radical hypothesis is that babies’ wariness toward plants is a remnant of an ancient survival mechanism.

For our ancestors, plants were both essential (a well-gathered meal, a well-thatched roof, etcetera), and deadly (an ill-advised snack in the woods, a mis-measured herbal remedy). But dangerous plants don’t announce themselves in any obvious way: They don’t gnash their teeth or flash their claws, and they definitely don’t chase you through the forest. For Wertz, this sets up “a really interesting learning problem.” You live in the midst of one leafy green thing after another, but you have no idea which ones are safe for dinner.

“If we only ate one thing, then you could imagine that maybe there would be some kind of program for identifying that specific thing. But our diets are very broad, and humans live in a wide range of environments. So each individual human, when they are born, has to solve the problem of which plants in this particular environment are the dangerous ones, and which are the edible ones.”

Wertz had read about the strategies that other creatures use to navigate this difficult cost-benefit analysis. There are simple physiological mechanisms that help: for example, chemicals in the guts of many animals will break down plant toxins. And there are more impressive behavioral mechanisms, too. Some animals have learned to merely nibble at plants they are trying for the first time. That way, even if the plant turns out to be hazardous, they’ve only ingested a Whole Foods-sized sample.

According to Wertz’s hypothesis, what wound up working best for early humans was what she calls a “social learning mechanism.” Hunter-gatherers mostly played it safe, avoiding plants until they could reliably figure out if they were edible or not. The most helpful clues, it turns out, came from each other: If Mom eats parsley, you can too.

For humans today, Wertz thinks something astonishingly similar applies. Babies are reluctant to engage with plants, until they pick up social cues from the people around them that indicate whether it’s okay to do so.

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Clare Sestanovich is a former story researcher for The Atlantic.

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