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An infant’s first laugh is among the most intoxicating moments in parenting. When each of my sons laughed for the first time, I felt like they’d come to life all over again. I felt like Descartes: It laughs; therefore it is. And then I reminded myself to stop calling the baby it. If you are the sort of parent who overthinks things, that first laugh is shadowed by a lot of questions: Why does he know that is funny? How does he even know what funny is? Humor was long conceptualized as taking place at a higher mental level, a level to which infants had not yet ascended. But that left a stubborn problem: No one had bothered to explain why babies kept laughing.

It turns out they are laughing at you. And with you. Thinking of humor in structural, analytical terms deprives us of understanding what babies can do: They may not understand the joke, but they can share in this glorious feeling of funniness. Laughter in infancy, as Vasudevi Reddy, a psychologist at University of Portsmouth in England, has argued, seems to be an “intrinsically social, even interpersonal” act.

Remarkably, even without any deeper conception of why a grown-up might find something funny, the laughter of babies has the same characteristics as adult laughter. Like us, they laugh at many different things, in many different ways, but they often laugh in the same way at the same sort of thing. When the baby in our house laughs, I almost always know why. This is what nearly all parents report, Reddy said. Laughter isn’t a mysterious sound. It seems to bubble up from a deep and early engagement in the social world.The very first laughs, at around 3 to 4 months of age, seem to be smiles that grew too big for the face: They erupt in sound. In “Baby Meets World,” I write about the science of smiling, the way that social smiles build on each other, crescendoing in meaning and magnitude. Laughter works in much the same way. Through face-to-face engagement, we teach our babies what to laugh at and when it makes sense to laugh. Through parent interviews and close observation of infants, Reddy has documented that this sort of social back-and-forth is the source of almost all early laughter. Even when babies laugh from tickling, or from someone blowing on their belly, it is the social stimulation – the imagonnagetchu – that sparks the laugh as much as the physical stimuli.

Babies are keenly perceptive of silliness and playfulness. “If you can distinguish the serious and the playful,” Reddy said, “at least you’re part of the way toward understanding this person understands that funny face to be funny rather than this person has just gone nuts. Or this person has an odd face.” That’s what enables babies, from an oddly early age, to be connoisseurs of slapstick.

Within several months after his first laughs, your baby wants to make you laugh – and he won’t wait until he can tell jokes. After eight months, infants begin to display what Reddy calls clowning – they explicitly try to get someone else to laugh. They don’t have the idea first, but they instantly catch on to the idea that something might be funny. “It starts in very simple things,” Reddy said. “You’re laughing because they’re splashing you, so they splash you some more. And the things they do to make you laugh become subtler and more clever and more sophisticated.”

In Reddy’s book “How Infants Know Minds,” she recounts the story of an 11-month-old who imitated her great-grandmother’s snoring face – and then, after everyone laughed, did it again, and then again, “deliberately, waiting for a response.” She kept doing it for days. Like any good comedian, Reddy wrote, she used what works, and for her, as for many of us, the experience of getting other people to laugh was addictive. “Funniness,” Reddy concluded, “exists only in relation.”

Nicholas Day’s book on the science and history of infancy, “Baby Meets World,” is available on amazon.com. His website is nicholasday.net.