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Two young bonobos exhibit the ape equivalent of the human laugh, a “play face,” which is accompanied by laugh-like panting sounds. Just as in humans, if one ape laughs others usually do as well, especially during wrestling and tickling games.

Frans de Waal

One morning, the principal’s voice sounded over the intercom of my high school with the shocking announcement that a popular teacher of French had just died in front of his class. Everyone fell silent. While the headmaster went on to explain that the teacher had suffered a heart attack, I couldn’t keep myself from a laughing fit. To this day, I feel embarrassed.

What is it about laughter that makes it unstoppable even if triggered by inappropriate circumstances? Extreme bouts of laughter are awkward: they involve loss of control, shedding of tears, gasping for air, leaning on others, and even wetting of pants while rolling on the floor! What weird trick has been played on our linguistic species, that we express ourselves with stupid “ha ha ha!” sounds? Why don’t we leave it at a cool “That was funny!”?

Philosophers who regard a sense of humor as one of humanity’s finest achievements may find it puzzling that it is expressed with the sort of crude abandon associated with mere animals. But laughter is an inborn, universal human trait, one that we share with our closest relatives, the apes (see “The Laughing Species,” December 2000-January 2001).

A Dutch primatologist, Jan van Hooff of Utrecht University, set out to learn under what circumstances great apes utter their hoarse, panting laughs, and concluded that ape laughter has to do with a playful attitude. It’s often a reaction to surprise or incongruity—as when a tiny infant chimp chases the group’s top male, who runs away “scared,” laughing all the while. This connection with surprise is still visible in children’s games such as peek-a-boo, or jokes marked by unexpected turns, which we save until the very end and appropriately call “punch lines.”

What intrigues me most about laughter, however, is how it spreads. It’s almost impossible not to laugh when everybody else is. There have been laughing epidemics that lasted months, in which no one could stop for long. There are laughing churches and laugh therapies based on the healing power of laughter. All because we love to laugh and can’t resist joining those laughing around us.

The infectiousness of laughter even works across species. Below my office window at the Yerkes Primate Center in Atlanta, Georgia, I often hear my chimps laugh during rough-and-tumble games, and cannot suppress a chuckle myself. It’s such a happy sound. Tickling and wrestling are the typical laugh triggers for apes, and probably the original ones for humans. The fact that tickling oneself is notoriously ineffective attests to its social significance. And when young apes put on their “play face” (as the laugh expression is known), their friends join in with the same expression as rapidly and easily as humans do with laughter.

Shared laughter is just one example of our primate sensitivity to others. We aren’t Robinson Crusoes, sitting on separate islands; we’re all interconnected, both bodily and emotionally. This may be an odd thing to say in the West, with its tradition of individualism and liberty, but members of the species Homo sapiens are easily swayed in one emotional direction or another by their fellows.

That is where empathy and sympathy start—with the synchronization of bodies—not in the higher regions of imagination, or in the ability to consciously reconstruct how we would feel if we were in someone else’s “shoes.” And yet empathy is often presented as a voluntary process, requiring role taking, higher cognition, and even language. Accordingly, most scholarly literature on empathy is completely human centered, never mentioning other animals. As if a capacity so visceral and pervasive could be anything other than biological! To counter such widespread views, I decided to investigate how chimpanzees relate to and learn from one another.