Humans are much more flexible in
their behavior than most other animals.
For example, we figure out what to eat in every environment where we
find ourselves. Other animals are not so
lucky. If they find themselves outside
of the environment in which they evolved, they can have great difficulty
finding food.
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 nutrition. 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 attention 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.
