What baby's smile means

By encouraging your baby to smile, you're helping her develop self-esteem. It lets her know that her feelings are important and that she can affect her environment. It's also important for her overall brain development.

Babies flash toothless grins from birth, but these smiles are random and spontaneous – triggered simply by neurons firing in the brain stem and unrelated to good moods. When babies doze off for naps, the smiles really come out; scientists think this is because the responsible motor cells nestle close to the region of the brainstem where REM sleep originates.

This early mouth curling is like a fake smile – the kind you need for passport photos or when pretending you like the outfit your husband dressed your child in. A genuine smile of delight or amusement comes directly from the limbic system (the brain’s emotional center) and it recruits eye muscles called the orbicularis oculi (squinting and raising up the cheeks). The catch is that you can’t force these muscles to work – they are under involuntary control, so only a sincere feeling makes for a truly beaming face.

Between four and ten weeks of life, the limbic system and motor networks are sufficiently mature to make for baby’s first emotional smile. Across all cultures, the social smile pops up at the same time. Even babies who are blind show us their grins of happiness on schedule; they just cue off of voices and touch rather than a familiar face.

This is the first time that many parents feel the tugs of a real relationship with their baby – no longer just the one-way caretaking of the early months, but an actual back-and-forth interaction. Smiles are one of the building blocks of attachment and, in fact, some psychologists think that even older babies save the genuine signs of joy (eyes and cheeks included) for their parents – strangers are more likely to get a “fake” smile.

I looked through our family videos for footage of my son’s first months and, sure enough, there I was, putting on the classic newborn show. Hovering twelve inches from his face, I had the high-pitched silly voices and goofy looks going – I was working overtime to make him chuckle. And research suggests this might be because I am “addicted” to my son’s smiles. When babies grin in amusement, reward centers in the parent’s brain light up. These areas – specifically the substantia nigra, the striatum, and emotional networks in the frontal lobes – use the neurotransmitter dopamine, which gives us a boost in mood and leaves us wanting more.

So smiling is more than just a pleasant perk of being human; it has been shaped by evolution to keep us together. Compared to other species, our infants are born very immature, needing especially devoted parents who are in it for the long haul. Bonding behaviors are programmed into our biology, and we instinctively know how to use them without much practice – like a dolphin swimming or a bird nesting. Truly loving smiles are unconscious and, with the help of some potent brain chemicals, they strengthen our relationships with our kids.