The fascinating reason you loved peek-a-boo

In Italy, it’s called il gioco del cucù. Palestinians say ba’ ’éno. And in Japan it’s inai-inai... ba! But in every language, the response is usually the same. Peek-a-boo is a near-universal source of laughter and connection for infants and adults— it’s the first game that almost everyone plays. So what is it about this goofy game that babies love so much? While infants develop at different rates, many cognitive and motor abilities emerge in a certain order. For example, while 6-month-olds can typically grab things— and then promptly drop them— walking and talking generally begin around 12 months. This timeline was first formally charted in 1936 by Swiss psychologist Jean Piaget. And while most modern researchers agree this process is more fluid than Piaget believed, the early developments he identified are key for understanding peek-a-boo. First is face processing. This happens almost immediately— newborns as young as two days old can recognize their caregivers’ faces. Then, around 6 to 10 weeks, babies will begin social smiling— this is when they notice nearby laughing or smiling faces and start to mimic them. At 2 to 4 months, they may begin to understand cause and effect. And finally, between 4 and 7 months they learn object permanence. Piaget described this as the understanding that people and objects continue to exist even when you can’t see them. So before this period, “out of sight” can literally mean “out of mind”— making hiding your face akin to a magic trick. Before object permanence, this disappearing act can range from confusing to pleasantly surprising. But by 9 months, these developments are working together for peak peek-a-boo performance. At this age, infants can focus on the game longer, predict the timing of the reveal, and even look for the hidden object or person. And since infants learn about the world through play, peek-a-boo is one of their first teachers. In one study, 11-month-olds were shown a barrier, the bottom of which was hidden behind a screen. Then researchers rolled balls and toy cars behind the screen, removing it afterwards to show that the toys had either stopped at the barrier as expected, or, somehow, gone through it. Infants shown these seemingly magical toys showed more interest in them afterwards— even ignoring new objects in favor of toys that challenged their expectations. The social expectations peek-a-boo develops may be even more important. The game features several hallmarks of what researchers call social play: eye contact, turn-taking, and joint attention. These skills are the foundation of human conversation, and since it's a conversation, what the adult does matters. Just as caregivers learn to read their infant’s cries and verbal cues, babies learn how adults respond to their behavior. Some play researchers call this back and forth “serve and return” interaction, and peek-a-boo is a prime example. This call and response structure is also why some psychologists describe peek-a-boo as a baby’s first joke. It’s an interaction where the format is reliable, but the content is surprising. As other motor and cognitive abilities develop, this foundation of social skills and object permanence informs various kinds of play. Once kids begin walking and talking, peek-a-boo usually evolves into hide-and-seek— though their hiding isn’t usually very good at this age due to poor impulse control and a lack of theory of mind. This is the ability to understand, imagine, and predict other people’s mental states. Without it, a child might think they’re hidden simply by covering their own eyes— after all, if they can’t see you, surely you can’t see them. Once theory of mind develops around age 3 or 4, kids can begin playing pretend together, all occupying a shared imaginary world. Though even the most cooperative pretend play is often sprinkled with peek-a-boo style surprises. By ages 5 and 6, language expands to inform more playful negotiation, leading to games with more complicated rules. After this point, most kids’ favorite types of play are more determined by their personalities and interests than their cognitive development. But no matter what play they pursue, as adults they’ll likely wind up playing peek-a-boo again—

this time from an all-new perspective.