Why do babies
suck their hands
so much?

The full narration of the video.

Xiaoningmeng asked: when a baby sucks their hands like crazy, are they hungry? Or building a bad habit? Grandparents often reach over and pull the hand out. Hold on: both guesses are only a little bit right. Let's go through the guesses one by one. First guess: hungry.

That one is partly true. When a newborn is hungry, they bring a hand to the mouth and turn their head, looking for milk. But even right after a feed, with a round full tummy, they keep sucking. Hunger is only one reason. Second guess: teething. But the first tooth usually shows up after six months. Lots of babies suck the hardest at two or three months.

Third guess: a bad habit. This one is the easiest to knock down. Doctors watched babies inside the womb on ultrasound. One study looked at fifty-five of them, between twenty and thirty-two weeks of pregnancy. Thirty-five were already sucking their fingers. Not born yet, nobody taught them, and they can already do it. It's not a bad habit they picked up. It's a skill they arrive with.

So what is that skill for? The answer has two layers. Layer one: sucking is a switch the baby can press all by themselves. Think of a light switch on the wall at home, installed before birth. Press it, the body relaxes, and the crying fades.

In studies: newborns given a pacifier cried for a much shorter time. Sucking a hand is the version that needs no grown-up. Layer two: at two to four months, the baby discovers: the switch is right there on their own hand. Before, the hands just waved around in front of their face. Now they can steer them: again and again, right to the mouth.

The U.S. CDC: its checklist for four-month-olds has one line: brings hands to mouth. So that wild sucking phase is practice for a new skill: making the hands obey. So how long do they keep pressing that switch?

Most kids, by two to four, learn other ways to calm down. Then they stop on their own. The American Academy of Pediatric Dentistry suggests: if a child still sucks every day after age three, have a dentist take a look. Here the switch idea breaks down: you can press a wall switch for a lifetime, but years of sucking can push teeth out of line.

In one U.S. study: kids who quit before age one had misaligned back teeth about six percent of the time. Those who kept going past four: more than one in five. For a little baby right now, there are just three things to do: keep the hands clean and the nails short. If they turn their head looking for milk, feed them. Don't tie the hands, and don't put anything bitter on the fingers. When in doubt, ask your doctor. Next time a crying baby stuffs a fist in their mouth, don't pull it out. Count: how many sucks until they settle?

Xiaoningmeng, how many do you guess?