Speakers, it turned out, were 46 percent more likely to laugh than listeners—and what they were laughing at, more often than not, wasn’t remotely funny. When Provine and his team of undergrads recorded the ostensible “punch lines” that triggered laughing in ordinary conversation, they found that only about 15 percent could be called humorous. In his book, Laughter: A Scientific Investigation, Provine lists some of the laugh-producing quotes: “I’ll see you guys later.” “Put those cigarettes away.” “I hope we all do well.” “It was nice meeting you too.” “We can handle this.” “I see your point.” “I should do that, but I’m too lazy!”

Not So Funny

Previous studies of laughter had assumed that laughing and humor were inextricably linked, but Provine’s early research suggested that the connection was only an occasional one. As his research progressed, Provine began to suspect that laughter was in fact about something else—not humor or gags or incongruity but our social interactions. He found support for this assumption in a study that had already been conducted, one analyzing people’s laughing patterns in social and solitary contexts. “You’re 30 times more likely to laugh when you’re with other people than you are when you’re alone—if you don’t count simulated social environments like laugh tracks on television,” Provine says. Think how rarely you’ll laugh out loud at a funny passage in a book but how quick you’ll be to give a friendly laugh when greeting an old acquaintance. Laughing is not an instinctive physical response to humor, the way a flinch is a response to pain or a shiver to cold. Humor is crafted to exploit a form of instinctive social bonding.

The more technical parts of Provine’s work—exploring the neuromuscular control of laughter and its relationship to the human and chimp respiratory systems—draw on his training at Washington University in St. Louis under Viktor Hamburger and Nobel laureate Rita Levi-Montalcini. But the most immediate way to grasp his insights into the evolution of laughter is to watch video footage of his informal fieldwork, which consists of Provine and a cameraman prowling Baltimore’s Inner Harbor, asking people to laugh for the camera.

Typically, Provine asks someone to laugh and they demur, look puzzled for a second, and say something like, “I can’t just laugh.” Then they turn to their friends or family, and the laughter rolls out of them as though it were as natural as breathing. At one point Provine stops two waste-disposal workers driving a golf cart loaded up with trash bags. When they fail to guffaw on cue, Provine asks them why they can’t muster up a chuckle. “Because you’re not funny,” one of them says. Then they turn to each other and share a hearty laugh. “See, you two just made each other laugh,” Provine says. “Yeah, well, we’re coworkers,” one of them replies. 

The insistent focus on laughter patterns has a strange effect on me as Provine runs through the footage. By the time we get to a cluster of high school kids, I’ve stopped hearing their spoken words at all, just the rhythmic peals of laughter breaking out every 10 seconds or so. After one particularly loud outbreak, Provine turns to me and says, “Now, do you think they’re all individually making a conscious decision to laugh?” He shakes his head dismissively. “Of course not. In fact, we’re often not even aware that we’re laughing. We’ve vastly overrated our conscious control of laughter.”

Uncontrollable giggles

The limits of our voluntary power over laughter are most clearly exposed in studies of stroke victims who suffer from a disturbing condition, known as central facial paralysis, that prevents them from voluntarily moving either the left or right side of their face, depending on the location of the neurological damage. When these individuals are asked to smile or laugh on command, they produce lopsided grins: One side of the mouth curls up, the other remains frozen. But when they’re told a joke or they’re tickled, traditional smiles and laughs animate their entire faces.

There is evidence that the physical mechanism of laughter itself is generated in the brain stem, the most ancient region of the nervous system, which is also responsible for fundamental functions like breathing. Sometimes called the reptilian brain because its basic structure dates back to our reptile ancestors, the brain stem is largely devoted to our most primal instincts, far removed from the complex, higher-brain skills that allow us to understand humor. And yet somehow, in this primitive region, we find the urge to laugh.

Why do we have this unconscious propensity for something as frivolous as laughter? As I watch them on the screen, Provine’s teenagers remind me of an old Carl Sagan riff, which begins with his describing “a species of primate” that likes to gather in packs of 50 or 60 individuals, cram together in a darkened cave, and hyperventilate in unison, to the point of almost passing out. The behavior is described in such a way as to make it sound exotic and somewhat foolish, like salmon swimming furiously upstream to their deaths. The joke, of course, is that the primate is Homo sapiens and the group hyperventilation is our laughing together at comedy clubs or in theaters—or with the virtual crowds of television laugh tracks. I’m thinking about the Sagan quote when another burst of laughter arrives through the TV speakers, and without realizing what I’m doing, I find myself laughing along with the kids on the screen. I can’t help it. Their laughter is contagious.

Tickle me Washoe

We may be the only species on the planet that laughs together in such large groups, but we are not alone in our appetite for laughter. Not surprisingly, our near relatives, the chimpanzees, are also avid laughers, although differences in their vocal apparatus cause their laughter to sound somewhat more like panting. “The chimpanzee’s laughter is rapid and breathy, whereas ours is punctuated with glottal stops,” says legendary chimp researcher Roger Fouts. “Also, the chimpanzee laughter occurs on the inhale and exhale, while ours is primarily done on our exhales.”

Chimps don’t do stand-up routines, of course, but they do share a laugh-related obsession with humans, one that Provine believes is central to the roots of laughter itself: Chimps love tickling. Back in his lab, Provine shows me video footage of a pair of young chimps named Josh and Lizzie playing with a human caretaker. “That’s chimpanzee laughter you’re hearing,” Provine says. It’s close enough to human laughter that I find myself chuckling along.

Parents will testify that ticklefests are often the first elaborate play routine they engage in with their children and one of the most reliable laugh inducers. According to Fouts, who helped teach sign language to Washoe, perhaps the world’s most famous chimpanzee, the practice is just as common, and perhaps more long-lived, among the chimps. “Tickling . . . seems to be very important to chimpanzees because it continues throughout their lives,” he says. Even at the age of 41, Washoe still enjoys tickling and being tickled. Among young chimpanzees who have been taught sign language, tickling is a frequent topic of conversation.

Like laughter, tickling is almost by definition a social activity. And like the incongruity theory of humor, it relies on a certain element of surprise, which is why it’s impossible to tickle yourself. A number of tickle-related studies have convincingly shown that tickling exploits the sensorimotor system’s awareness of the difference between self and other: If the system orders your hand to move toward your belly, it doesn’t register surprise when the nerve endings there report being stroked. But if the touch is being generated by another sensorimotor system, the belly stroking will come as a surprise.

Jared Diamond wrote a short book with the provocative title Why Is Sex Fun? that suggests an evolutionary answer to the question of why tickling is fun: It encourages us to play well with others. In his book, Provine suggests that feigned tickling can be thought of as the original joke, the first deliberate behavior designed to exploit the tickling-laughter circuit. Our comedy clubs and our sitcoms are culturally enhanced versions of those original playful childhood exchanges. Along with the suckling and smiling instincts, the laughter of tickling evolved as a way of cementing the bond between parents and children, laying the foundation for a behavior that carries over into the social lives of adults. If we once laughed at the surprise touch of a parent or sibling, we now laugh at the surprise twist of a punch line.

Playing is what young mammals do, and in humans and chimpanzees, laughter is the way the brain expresses the pleasure of that play. “Since laughter seems to be ritualized panting, basically what you do in laughing is replicate the sound of rough-and-tumble play,” Provine says. “And you know, that’s where I think it came from. Touching and being touched is an important part of what it means to be a mammal.”

There is much we don’t know yet about the neurological underpinnings of laughter. We do not yet know precisely why laughing feels so good; one recent study detected evidence that stimulating the nucleus accumbens, one of the brain’s pleasure centers, triggers laughter. Some anecdotal and clinical evidence suggests that laughing makes you healthier by suppressing stress hormones and elevating immune-system antibodies. If you think of laughter as being basically synonymous with the detection of humor, the laughing-makes-you-healthier premise seems bizarre. Why would natural selection make our immune system respond to jokes? Provine’s approach helps solve the mystery. Our bodies aren’t responding to punch lines; they’re responding to social connection. And even if we don’t yet understand the neurological basis of the pleasure that laughing brings us, it makes sense that we should seek out the connectedness of infectious laughter. We are social animals, after all. And if that laughter often involves some pretty childish behavior, so be it. “This is why we’re not like lizards,” Provine says, holding the Tickle Me Elmo doll on his lap. “Lizards don’t play; they’re not social the way we are. When you start to see play, you’re starting to see mammals. So when we get together and have a good time and laugh, we’re going back to our roots. It’s ironic in a way: Some of the things that give us the most pleasure are really the most ancient.” 

Discover ran an earlier version of this article in 2005.