Every human smiles. And everyone knows exactly what a smile means. This universality makes many people think that smiling evolved. But why?
What was the benefit that made smilers more successful? So successful that their smiley genes were passed on so much that now we all do it.
Some think that smiling may have been a "signal" to others. But just what information did it communicate? And why was passing on that information so helpful?
Smiling as a signal
Smiling can convey a lot of information about an individual. As such, many researchers think it evolved as a sort of signal. But which piece of information was it beneficial to pass on?
After all, smiling can tell you a lot of less-than-desirable things about someone. Like how many teeth they have or - in some rather disgusting examples - what they had for breakfast. It's easy to conjure up "just so" stories that show how any of these bits of information could drive the evolution of a smile. But by making up this story we'd be the worst kind of evolutionary psychologist. Which is literally the worst kind of person.
But whilst it might be hard to figure out exactly why people smile, it's easy to show that smiling is a form of signalling. Evidence for this comes from the fact it only really takes place in social circumstances, when it could convey information to others.
For instance, one fascinating study on people bowling showed that there wasn't a particularly strong correlation between smiling and something good happening (like bowling a strike). Instead, whether someone was socially engaged was a much better predictor of whether they would smile. Similarly, people at a hockey game were much more likely to smile when they were socially involved, rather than just watching the game (although these results could be influenced by the fact hockey is just a bit rubbish; hardly worth smiling at).
Faking a smile
However, signals can be bad. At least; for the person watching them. Signals could convey false information, deceiving them, and allowing for you to be manipulated.
And this isn't just a problem with smiling. Any signal has the potential for this sort of abuse. As such, humans have developed a whole host of elaborate ways of avoiding "fake" signals. For example, whilst many people find a deep voice cool and manly; someone putting on a deep voice gains none of those cool points. We're great at spotting the fakers and not giving into their fabricated voices.
One big trick for catching out cheats is by only paying attention to signals that are hard to fake. Peacocks judge each others' tails, since they can't really be fabricated ( unless you're a sneaky scientist). But that isn't really the case for smiling; which only requires a handful of muscles to be moved. Hardly a difficult thing to fake. Alternatively, if a signal takes a lot of effort then it's likely genuine; since it wouldn't be worth it to fake it. But again, smiling doesn't fall into that category either.
Instead, it might be the universality of smiling that makes it reliable. Similar things make just about everyone grin. Sharing resources and becoming pals will make you smile, regardless of who you are. Thus, if a smile is honest then both participant should be smiling (or at least, understand why the other person is smiling). When the emotions of both people don't line up is when something fishy is going on.
There hasn't been much research into that last idea; so treat it as a hypothesis. However, I would like there to be a real justification for my apathy. It would make me happy (or not).
So why smile?
All of this leads us back to that first question. Why do we smile? Well, we know it's signalling something. And those signals could be kept honest by the fact we all agree on what's worth a smiley signal.
The honesty behind the smile could hold the answer. Both parties agree on what's worth smiling about. So a smile becomes a quick, genuine way of saying "I like this." This could serve to encourage you to co-operate and reach the goals worth smiling over.
As I said before, there hasn't been too much research in that direction so we can't actually say if that's the case. On the plus side, that does mean there's no dumb EvoPsych research on the subject I need to call bullshit on.
Such a nice change.
tl;dr
We don't really know why humans smile. But we do know that it signals something and could be kept honest through sympathy.
References
Dezecache, G., Mercier, H. and Scott-Phillips, T.C., 2013. An evolutionary approach to emotional communication. Journal of Pragmatics, 59, pp.221-233.
Kraut, R.E. and Johnston, R.E., 1979. Social and emotional messages of smiling: An ethological approach. Journal of personality and social psychology, 37(9), p.1539.
McCullough, M.E. and Reed, L.I., 2016. What the face communicates: clearing the conceptual ground. Current Opinion in Psychology, 7, pp.110-114.