Perhaps we picked up smiling from our ape-like ancestors; perhaps it's cultural. And if you ask a neuroscientist why babies wait a few months to smile, you'll get several answers.

Child Development

Why Don't Newborn Babies Smile? Why Do They Look So Worried?

by Joshua A. Krisch

My two-month-old just started smiling, and her grins are infectious. But they’re scientifically confusing. Did she pick it up from watching us smile at her? Probably not. Blind babies smile, too. Is her smile merely an involuntary curl of the lips? Also unlikely. She’s now performing what psychologists call “social smiling” — when a child grins in response to an adult’s smile.

So what is a smile — and why do newborns take a few months to get the hang of it?

Hypotheses abound, but there’s no one comprehensive theory. Some suspect we picked up smiling from our ape-like ancestors; others maintain that it’s largely cultural. And it’s similarly unclear why babies start smiling late in the game (although neuroscientists have their theories). Here’s everything we know about the science of smiling — and what remains a mystery:

A Short History Of Smile Science

Charles Darwin was among the first to take a stab at explaining why humans smile. He noted that many animals warn one another off by baring their teeth, and suggested that early humans may have routinely greeted strangers with a canine snarl. Over time, Darwin speculated, this greeting lost its edge and became one way we recognize the presence of another person.

But it was one of Darwin’s less well-known contemporaries, Guillaume-Benjamin Duchenne, who contributed the first meaningful scientific take on smiles. Duchenne mapped the differences between genuine smiles and that expression you make when you get socks for your birthday, giving rise to what contemporary scientists refer to as fake non-Duchenne smiles and genuine Duchenne smiles (the real deal exercises the muscles around your eyes, not just your mouth).

From Duchenne until now, there hasn’t been much in the literature. Scientists have noted that non-human primates display something akin to a smile that’s associated with camaraderie. And a rash of research has examined how smiling differs across cultures. One study demonstrated that Americans and Australians can guess the nationality of a smiling Caucasian person, but cannot do so when the Caucasian person makes a neutral expression; other studies have confirmed that diverse cultures have different rules governing when it’s appropriate to smile.

“Rarely do we think, ‘Isn’t it interesting that another culture has different smiling rules?’ We view them as being a different type of person,” psychologist Marianne LaFrance of Yale University, told Wired. “At home, judgments based on a person’s smiling habits might be warranted. But when you’re talking about cross-cultural boundaries, those judgments can be really off-base.”

An Anthropological Approach To Smiling

Where biologists left off, anthropologists have taken over. Anthony Stocks, professor of anthropology at Idaho State University, told Scientific American that there’s an important difference between tooth-baring smiles and less toothy smiles. When a baby smiles with his or her teeth open, this indicates fright and may indeed be an involuntary throwback to how our hairier ancestors snarled at threats. “On the other hand, a non-toothy, not-so-broad-but-open-lipped smile is associated with pleasure in human infants,” he said. “Somehow we seem to have taken the fright-threat sort of smile and extended it to strangers as a presumably friendly smile.”

Frank McAndrew, who studies facial expressions at Knox College, adds that even primates sometimes smile in submission. “Showing the teeth, especially teeth held together, is almost always a sign of submission,” he told Scientific American. “The human smile probably has evolved from that.” The difference, he says, is in whether your lips are curled back and your teeth are ready to bite, or whether your lips are relaxed and your teeth pressed together.

A Neuroscientific Approach To Smiling

If our goal is to discover the origins of human smiling, neuroscience isn’t going to get us very far. “While neuroimaging data (pictures of the brain when it is active) tells us how emotional expressions are perceived, it doesn’t tell us why we smile (as opposed to frown, for example),” psychologist Nakia Gordon of Marquette University told her school’s magazine.

But neuroscience may provide hints as to why babies start smiling only after a couple months of straight-faced cooing. Brain imaging studies suggest that we cannot be truly happy unless we’re capable of self-referential thinking, Dustin Scheinost of Yale University’s Child Study Center told Scientific American. And studies have shown that building that sort of brainpower takes time. “To be happy, you have to know that you’re happy,” Scheinost says. “A lot of unhappiness initially isn’t really unhappiness but rather low-level feelings like ‘I’m hungry’.” After a few months, studies suggest, babies’ brains have developed enough to know that “I’m unhappy because I’m hungry.” At this point, you can expect your baby to start smiling when he or she is happy.

Another prerequisite for happiness is memory — if you cannot retrieve any happy memories, you’re unlikely to smile very often. A 2014 study published in Science found that a stable network of neurons is required for memory to form, and that infants’ growing brains shake up their neuronal networks so often that memories seldom form. (Which, given the indignities of early childhood, isn’t necessarily a bad thing). Perhaps babies don’t smile until their brains calm down a bit and the harrowing process of neurogenesis gives them time to develop their smirks.

How To Use Smile Science To Make Yourself Happy

We may not know where smiling comes from — or why babies take a while to catch on — but how smiling affects the smiler is well-established science. In a word, you’re happier when you’re smiling, even if your smile is insincere, and decidedly non-Duchenne. One clever illustration of this principle appeared in The Journal of Personality and Social Psychology in 1988. All participants were asked to read comics and rate their funniness, but some were asked to hold a pencil between their teeth — forcing their faces into awkward smiles. Those forced into smiling rated the comics significantly funnier, demonstrating that smiles can influence our feelings.

So I’ll keep on smiling at my two-month-old, and I suspect she’ll keep on smiling back. I have no clue whether she picked it up from me or from baboon teeth-baring behaviors, and I can’t say for sure whether she’s now self-referentially happy or finally capable of remembering happy moments. But one thing seems clear — we’re both happier when we’re smiling.