Today’s Factismal: When a baby smiles with its teeth apart and showing, the baby is frightened. When a baby smiles with its teeth hidden, the baby is happy.
The human smile is a puzzling thing. We know that we do it when we are happy, but why do we do it? And why do we smile with our teeth together when most other primates smile with them apart? And when does this behavior start? Is it programmed into us by society or is it more intrinsic?
It turns out that when a typical primate smiles with its teeth apart and gums pulled up to show off the teeth, it is usually (but not always) showing aggression. And when it smiles with its teeth together or the lips down to hide the teeth, it is showing submission. The only exception to this behavior appears to be what primatologists call “rapid facial mimicry” and what parents call “making faces”. Two or more primates will grimace at each other, trying to match the other’s face, as a way of becoming better friends. (No word on if their faces ever freeze that way.)
Being primates ourselves, humans exhibit many of the same behaviors. But society always adds a veneer of confusion onto the basic data, which is why scientists who study the evolution of human behavior like to watch babies – they haven’t been as influenced by social norms and show a purer response. And they’ve found that human babies tend to follow the primate rule: teeth together and gums hidden, happy baby; teeth apart and gums showing, unhappy baby. And it isn’t just babies that follow these rules; the same pattern has been observed in blind people who have never had an opportunity to see others smile.
Of course, babies do more than smile; they also laugh. And that’s another rich field for research (and a little squee). If you happen to be the parent of a young baby, then the folks at the Baby Laughter Project would like your help in finding out why babies laugh and what that tells us about how our brains develop. If you’d like to help (or just want to watch videos of laughing babies), then head over to