The flexibility of human behavior comes at a cost. Ultimately, we have to learn how to navigate our environment rather than having a lot of that information pre-wired into the system. That learning is effortful and potentially dangerous.

Consider the problem of eating plants. Many plants are edible and are important sources of . But, some plants are not things we can digest and—worse yet—some are poisonous.

A fascinating paper by Annie Wertz and Karen Wynn in the April, 2014 issue of Psychological Science examines infants’ ability to learn about what plants are edible. Infants clearly don’t come wired to know which plants are edible, but their research suggests that infants may come wired to pay to the edibility of plants.

In one experiment, 18-month-olds watched an experimenter perform a series of actions. The experimenter first took a fruit (say a dried apricot) off a realistic looking plant and placed the tip of it in his mouth and said “Hmmmmmm.” Then, he took a different fruit (say a dried plum) off an object shaped like a plant that was painted silver and housed in a glass case and did the same thing. So, one object looked like a plant, while the other did not. (Other children in this study saw the experimenter do the action on the object first and then the plant, so the order in which the actions were performed did not affect the results.)

After seeing these actions, the experimenter took other fruits off the plant and the object. Then, a second experimenter came in and asked the child which one they could eat. Children overwhelmingly chose the fruit that came from the plant.

The experimenters also ran three control conditions. In one, when the experimenter took the fruit off, he put it behind his ear rather than in his mouth. In the test, the infants were asked which object they could use. In this case, the children had no preference for the fruit from the plant over the fruit from the object.

Of course, it could just be that the plant was more familiar than the object. In another control condition, the plant was compared to a set of shelves. Most infants are used to seeing food taken from shelves in their home. In this condition, after seeing the fruits from the plant and the shelf put in the experimenter’s mouth, the infants strongly preferred to choose the fruit that came from the plant.

In a third condition, the infants saw the experimenter just look at the plant and say “Hmmmmmmm” and then look at the object and say “Hmmmmmmmm.” This condition was designed to test whether children simply had a preference for fruits that come from a plant rather than fruits that come from an object. In this case, the infants were equally likely to choose the fruits that came from the plant or the object. This condition is important, because it is potentially dangerous for infants to learn that all plants are edible, because some are dangerous.

Finally, the researchers also examined whether even younger infants might show this preference. In a final study, these same actions were shown to six-month-old infants. Six-month-olds are too young to choose for themselves. So, after the first experimenter took the fruits off the plant and the object, a second experimenter put each fruit in his mouth in turn and held there. The experimenters measured how long the infants looked at these events. Lots of work with infants shows that for unfamiliar situations, infants look longer at surprising events than at unsurprising events.

In this study, when the infants saw the first experimenter put the fruits in his mouth, they looked longer when the second experimenter put the fruit from the object in his mouth than when the experimenter put the fruit from the plant in his mouth. But, when the first experimenter put the fruits behind his ear, the infants looked for the same amount of time when the second experimenter put the fruits behind his ear, regardless of whether they came from the object or the plant.

This set of results suggests that by six-months of age, infants are ready to learn about which plants are edible. Evolution has not pre-wired humans with knowledge of specific plants that we can eat. Instead, we are wired to learn about plants from other adults. That mechanism is important for helping us to survive in a wide variety of environments.

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