Humor has always been subjective. Where some people prefer the physical comedy of Jim Carrey, others may find the dry approach of Albert Brooks preferable.

Babies, of course, are not big on subtlety. Making silly faces, tickling their feet, or pretending you’ve disappeared in an endless loop of temporal displacement—otherwise known as peek-a-boo—are all ways to get tiny people to laugh. But why exactly are they laughing? Is it because they find their parents funny, or it is a reflex? Are they processing humor, or is it merely a way of socializing? Is baby laughter a way of saying, “Please continue paying attention to me”?

We asked several babies and got no answers. (In many cases, a loaded diaper ended questioning prematurely.) Fortunately, a number of researchers have explored the topic of what infants find funny in depth. “Almost all babies are laughing by the time they’re 4 months old,” Gina Mireault, a professor of psychology in the behavioral sciences department at Northern Vermont University, tells Mental Floss.

But, Mireault adds, in both adults and babies, “one of the biggest misunderstandings about humor is that it doesn’t have to have anything to do with something being funny.”

From Smiling to Laughing

Early in their lives, babies are nonverbal—they burble and coo incoherent nonsense. Smiling, laughing, and crying are therefore crucial to interacting with the rest of the world. You probably won’t catch a baby laughing all by herself. “It’s a social response,” Mireault says. “The requisite ingredient is another person.”

Babies typically start smiling at 6 to 12 weeks old. At 3 to 4 months, babies will laugh at actions involving physical stimulation, like tickling, raspberries, or being bounced on their caregiver’s knee. (In a 2014 study conducted by developmental psychologist Caspar Addyman at the Goldsmiths InfantLab at the University of London, parents reported that tickling was the top way of getting their child to laugh.) At 5 to 6 months, they’ve learned enough about the world around them to comprehend the basic tenet of humor—a distortion of reality. Once they’ve grasped the appearance of normal human behavior, they’ll giggle at big eyes, blown-out cheeks, and high-pitched voices.

“It tends to involve behavior my colleagues have described as ‘clowning,’” Mireault says. “It could be a giant hat, a big bow tie, unusual voices, or walking in a funny way.” In observing these social rule violations, babies are amused because they have expectations about how people normally behave. You pretending to be a kangaroo is not what they anticipate.

“Surprise is one of the key elements of humor,” Mireault says. “There are two theories. One is called the Arousal-Safety hypothesis, and another is called the Benign Violation Theory. They’re the same thing. The idea is that humor that involves a surprise is perceived as non-threatening.”

If you hand your child a stuffed animal, they will expect the stuffed animal. If you suddenly toss it on the floor, this new development will probably cause them to laugh. The key is familiarity and a sense of playfulness. A smiling adult tossing it aside will be funny. A stranger angrily throwing it against a wall will not. Even Charles Darwin made note of this, writing in his 1872 work, The Expression of the Emotions in Man and Animals, that being tickled by a stranger would prompt a child to “scream in fear.”

There are exceptions. In London, children’s theater producers Sarah Argent and Kevin Lewis have staged several plays aimed at a baby demographic aged 6 months and older with assistance from Caspar Addyman and research from the InfantLab. Plays like Out of the Blue and Shake, Rattle, and Roll anticipated a baby’s natural wariness of strangers. Performer Maisie Whitehead “met” the babies at the start of Shake, Rattle, and Roll, singing to them to get them accustomed to her presence. During the show, Whitehead pretending to “wobble,” losing and regaining her balance, entranced the babies.

What stood out for Lewis was the idea that a baby’s laugh can be a method of control. “There is a sense that laughter is a tool of power,” Lewis tells Mental Floss. “’If I laugh, the big person does this thing or action again. I can keep making them do it again and again and again and again and keep having pleasure and I am in control!’” In laughing, babies are utilizing a form of manipulation.

Why Babies Love Peek-a-Boo

Babies can also pick up on intention. If you pretend to drop something and say “ha, ha,” the baby will probably laugh. But if you say “uh-oh” and seem concerned, the baby will treat it like a serious event. Dropping objects with an exaggerated response was, in fact, a highlight of Argent's productions, sending babies into giggle fits.

There’s an expiration date on such actions, since babies will quickly learn the unexpected action—tossing the stuffed animal—can now be expected. Or, as Mireault puts it: “Babies aren’t stupid. They won’t find it infinitely funny.”

Bust out a game of peek-a-boo and watch your baby get the giggles.Brand X Pictures/iStock via Getty Images

The more babies learn about the world, the more caregivers are able to subvert their expectations. But one misunderstanding is what leads to the closest thing to a guaranteed laugh parents can perform without resorting to the cheap trick of tickling: peek-a-boo.

There are a couple of reasons why it works. One relates to a construct identified by Swiss psychologist Jean Piaget called object permanence, or the idea something that’s out of sight still exists.

To a baby, the existence of a physical object depends on whether it’s visible. “If you hide something from a 6-month-old baby, like car keys, by covering them up, the baby won’t look for them," Mireault says. The idea the keys have been obscured is not a concept they can grasp.

The same holds true for faces. If a parent covers their face with their hands, its sudden reappearance is a delightful surprise. Adds Mireault, “When you pop up again, it’s like, ‘Holy smokes, where did you come from?’”

Peek-a-boo is also a baby pleaser because it involves that distortion of reality—big eyes and silly faces. By 8 or 9 months, however, babies have caught on to object permanence. Now, when a parent leaves the room, the baby might get upset. They know the adult is in another room, as opposed to just having briefly blipped out of existence.

That doesn’t mean peek-a-boo will no longer be effective. They can be amused by the fulfillment of the expectation—that their parent resurfaces—or by hiding themselves. Alternately, a baby may laugh, but as they get older, it may become less sincere. “Babies can fake laugh at 6 months," Mireault says. "They can laugh to get attention. They start to babble and get vocal control and use voluntary laughter. Like when someone says, ‘Ha, ha, ha,’ and it doesn’t sound super genuine. It’s just a nice social gesture.”

It is disturbing to think that an adult considered a comic genius by a baby at 4 months may be a Vegas hack by 6 months, deserving only sympathy chuckles. By laughing, Mireault says, “The infant is saying, ‘Look, this is not that funny anymore, but I don’t want to stop playing, so come up with something else.’”

Babies' Sense of Humor

As babies age into toddlerhood, other kinds of humor begin to make sense. At 7 to 9 months, they find humor in contradictions. Put a hat on a dog and watch them crack up. At 12 months, they may see a cup and call it a spoon or laugh at an adult who does the same thing.

But the real landmark in a baby’s sense of humor may come when they make an attempt to make others laugh. At 5 months, enough motor control has been established so that babies can begin to hold up their own smelly feet rather than wait for a parent to do it. At 8 months, they’ll tease others. “It shows they understand other people can be tricked," Mireault says. "They’ll offer something like car keys and when you reach for them, they’ll pull them away.”

This is more than just humor. It shows a theory of mind, or the idea that other people have different thoughts, beliefs, and expectations. Funny faces may seem simple, but humor is a cognitive puzzle that helps babies grow up—and eventually keep the car keys.