Smiling seems like a pretty straight forward way to communicate to people that you're happy, or that you want to appear so. Less straightforward is the way that smiling affects you.
Have you ever met someone with what you might call a 'fake' smile? You might have heard the expression before about someone whose smile doesn’t reach their eyes. Often this is apparent on the face of our customer service representatives; when we order food from 'chirpy' restaurant staff or are greeted by receptionists. That’s because there are, in fact, two kinds of smile.
Paul Ekman and Mark Frank did a great deal of research into this in the mid-1990s. They looked into something called a ‘Duchenne smile’; named after a long-dead French researcher who first discovered that when we activate muscles in our eyes and in our mouth when we smile out of genuine happiness. He discovered this by electrocuting people. Ekman and Frank used far less exciting means to find out far more exciting results, including the specific muscles involved in a genuine vs non-genuine smile. When these are activated, we percieve the genuineness, and when they aren't, we detect deception. Perhaps more interestingly, we apparently find people who are smiling those genuine Duchenne smiles more attractive.
More interestingly again, a little earlier than Ekman and Frank, this study demonstrated a truly subtle feature of smiling. If you were to activate the muscles involved in a smile by something as unrelated to smiling as, say, holding a pencil in your mouth, you'll find cartoons funnier than when the muscles are left alone and even more so again than those holding the pencil in such a way as to activate frowning muscles.
This effect appears to be far more significant when making an already positive experience more positive than for making a negative one more positive, but has an impact either way. Most recently, this feedback loop has been shown to have an effect on our physical stress levels to boot.
The opposite seems to be true of frowning. As I mentioned in the last paragraph, activating frowning muscles while watching cartoons makes them less amusing. But it will also make pain more painful. And if you inhibit our ability to frown, for example with an injection of botox, you're likely to report more happiness than frown-capable people.
It's all very bizarre. But, we know that emotions are complete bodily experiences, with many vectors of influence and not always from the obvious places. Still strange that such a small thing can have such a large effect.