I used to think my baby smiled because he was happy.

This was last year. Isaiah, once he hit 2 months, smiled like he was trying to catch flies—mouth open wide, eyes crinkled, tongue hanging out. He ended up snagging strangers who were expectant grandparents instead. He wasn’t a happy baby; he was a caricature of a happy baby.

Then I took up the habit of reading developmental psychology journals and discovered that the strangers who cooed “Such a happy baby” might be missing the point. I might be missing the point. How and why infants smile is a subject that seems ripped from academic satire but turns out to be strangely gripping. And the research suggests that infant smiling, at least after the first six months, may be every bit as social as it is emotional. Smiling may be how we begin to communicate about the world outside of us, rather than how we express our inner state.

I’d always thought of smiling as a window into Isaiah’s world. But it turned out he might be using it to climb into mine.

The long-dominant view of facial expressions, the discrete emotions theory, held that certain emotions were innate and expressed in clear, defined ways. Infant smiling was considered basically a reflection of inner emotion. You were happy; therefore, you smiled. “Smiling was seen as the expression of an emotion that was supposed to be built in,” says Susan Jones, a professor of psychology at Indiana University.

But research from the last couple of decades shows there’s more to it. Smiling typically develops around six to eight weeks, a time when a baby spends her days gazing at faces, and when her vision widens to take in the whole face, not just the eyes. It’s unclear if there’s any emotion embedded in these very early smiles or what they mean, if anything, to the infant. Daniel Messinger, a professor of psychology at the University of Miami, suspects that these first smiles teach infants the positive associations attached to a smile that we adults already feel. Learning to smile—and learning what’s meant by a smile—is a process, much like learning how to walk. “I take smiling to be a social signal,” Messinger says. “I really think that babies are learning what joy is by sharing it with someone else.” In other words, smiling might not be so much an expression of a preexisting state as a path we take to get to that state.

Developmental psychologists have tracked, frame-by-frame, how infants have many different smiles, which they deploy in different circumstances. Babies smile with raised cheeks and pinched eyes—so-called Duchenne smiles, named for the 19th century French physician who used the severed heads of executed criminals to deduce the musculature of smiling—when they are especially focused and emotionally engaged. Mouth-open smiles are more likely to be playful. Simple smiles—no cheek-raising, no mouth opening—are cautionary steps toward interaction.

Once they master the variations, infants develop early smiling exchanges into more sophisticated forms of communication. They coordinate their gaze with someone else, an achievement that’s called joint attention. Its onset, which is the subject of enormous interest and research in contemporary psychology, is when we begin to have goals and tasks in tandem with others—a developmental tipping point.

Babies initiate joint attention by smiling at an object—a new toy, say—and then looking at a person while maintaining that smile. These “anticipatory smiles” generally begin to appear between 8 months and 12 months. For Isaiah, a lot of those smiles involved the pig-with-wings weathervane down the street—definitely the sort of thing you’d want to consult with someone else on.

There’s growing evidence that these anticipatory smiles are a crucial moment in infant development—the moment when babies begin to be aware of their social world. They mark a new and unique cognitive step: sharing an emotion with someone else that’s about some third thing (a pig-with-wings weathervane, naturally).

An anticipatory smile doesn’t reflect just any emotion; it reflects a positive emotion. And somewhat amazingly, a baby’s level of anticipatory smiling—her interest in sharing positive experiences—seems to predict her future social behavior. A longitudinal study published last year measured anticipatory smiling in a group of infants at several points throughout their early childhood. Remarkably, levels of anticipatory smiling at 9 months of age successfully predicted social expressivity and competence—essentially, the ability and inclination to engage with other people—at 30 months, almost two years later.

Looked at from this perspective, the experience of smiling prepares us to participate in society. As Messinger has written, “Affecting and being affected by the positive emotional expression of the parent may lead infants to experience the happiness of others as essential to their own happiness.” It’s a powerful, moving idea, and as I considered it, I reframed how I thought about Isaiah’s smiles. He might not be smiling because he was happy. He might be smiling because I was happy. And, in due time, that just might make him happy, too.

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