How does one chimp imitate another? Does he identify with the other and absorb its body movements? Or could it be that he doesn’t need the other and instead focuses on the problem faced by the other? This can be tested by having a chimpanzee show another how to open a puzzle box with goodies inside. Maybe all that the watching ape needs to understand is how the thing works. He may notice that the door slides to the side or that something needs to be lifted up. The first kind of imitation involves reenactment of observed manipulations; the second merely requires technical know-how.

Thanks to ingenious studies in which chimps were presented with a so-called ghost box, we know which of these two explanations is correct. A ghost box derives its name from the fact that it magically opens and closes by itself so that no actor is needed. If technical know-how were all that mattered, such a box should suffice. But in fact, letting chimps watch a ghost box until they’re bored to death—with its various parts moving and producing rewards hundreds of times—doesn’t teach them anything.

To learn from others, apes need to see actual fellow apes: Imitation requires identification with a body of flesh and blood. We’re beginning to realize how much human and animal cognition runs via the body. Instead of our brain being like a little computer that orders the body around, the body-brain relation is a two-way street. The body produces internal sensations and communicates with other bodies, out of which we construct social connections and an appreciation of the surrounding reality. Bodies insert themselves into everything we perceive or think. Did you know, for example, that physical condition colors perception? The same hill is assessed as steeper, just from looking at it, by a tired person than by a well-rested one. An outdoor target is judged as farther away than it really is by a person burdened with a heavy backpack than by one without it.

Or ask a pianist to pick out his own performance from among others he’s listening to. Even if this is a new piece that the pianist has performed only once, in silence (on an electronic piano and without headphones on), he will be able to recognize his own play. While listening, he probably re-creates in his head the sort of bodily sensations that accompany an actual performance. He feels the closest match listening to himself, thus recognizing himself through his body as much as through his ears.

The field of “embodied” cognition is still very much in its infancy but has profound implications for how we look at human relations. We involuntarily enter the bodies of those around us so that their movements and emotions echo within us as if they’re our own. This is what allows us, or other primates, to re-create what we have seen others do. Body mapping is mostly hidden and unconscious, but sometimes it “slips out,” such as when parents make chewing mouth movements while spoon-feeding their baby. They can’t help but act the way they feel their baby ought to. Similarly, parents watching a singing performance of their child often get completely into it, mouthing every word. I myself still remember as a boy standing on the sidelines of soccer games and involuntarily making kicking or jumping moves each time someone I was cheering for got the ball.

The same can be seen in animals, as illustrated in an old black-and-white photograph from Wolfgang Köhler’s classic tool-use studies on chimpanzees. One ape, Grande, stands on boxes that she has stacked up to reach bananas hung from the ceiling, while Sultan watches intently. Even though Sultan sits at a distance, he raises his arm in precise synchrony with Grande’s grasping movement. Another example comes from a chimpanzee filmed while using a heavy rock as a hammer to crack nuts. The actor is being observed by a younger ape, who swings his own (empty) hand down in sync every time the first one strikes the nut. Body mapping provides a great shortcut to imitation.

When I see synchrony and mimicry—whether it concerns yawning, laughing, dancing, or aping—I see social connection and bonding. I see an old herd instinct that has been taken up a notch. It goes beyond the tendency of a mass of individuals galloping in the same direction, crossing the river at the same time. The new level requires that one pay better attention to what others do and absorb how they do it. For example, I knew an old monkey matriarch with a curious drinking style. Instead of the typical slurping with her lips from the surface, she’d dip her entire underarm in the water, then lick the hair on her arm. Her children started doing the same, and then her grandchildren. The entire family was easy to recognize.

There is also the case of a male chimpanzee who had injured his fingers in a fight and hobbled around leaning on a bent wrist instead of his knuckles. Soon all of the young chimpanzees in the colony were walking the same way in single file behind the unlucky male. Like chameleons changing their color to match the environment, primates automatically copy their surroundings.

When I was a boy, my friends in the south of the Netherlands always ridiculed me when I came home from vacations in the north, where I played with boys from Amsterdam. They told me that I talked funny. Unconsciously, I’d return speaking a poor imitation of the harsh northern accent.

The way our bodies—including voice, mood, posture, and so on—are influenced by surrounding bodies is one of the mysteries of human existence, but one that provides the glue that holds entire societies together. It’s also one of the most underestimated phenomena, especially in disciplines that view humans as rational decisionmakers. Instead of each individual independently weighing the pros and cons of his or her own actions, we occupy nodes within a tight network that connects all of us in both body and mind.

Reprinted from The Age of Empathy: Nature’s Lessons for a Kinder Society. Copyright © 2009 by Frans de Waal, a primatologist at Emory University. Published by Harmony Books, a division of Random House.