Anyone remember the Batman animated series episode that had the Joker dosing people with venom that forced a creepy smile onto their faces and drove them insane? Turns out you don't need the venom. The smile alone may drive people crazy.
Professor Makoto Natsume at Osaka University practices psychiatry as well as teaching it. He noticed some of his patients, especially his female patients, were doing something really creepy - permanently, brilliantly smiling. They did this without faltering for a moment, and they did it while describing some of the most horrible things that had ever happened to them.
When he asked the women about it, they weren't aware they were smiling at all. And when he asked them why they might be smiling in the first place, they said they needed to smile for their work. The service industry requires unwavering smiles, and employees are told to train to smile the right way, and to keep smiling all the time while they are at work. The women reported headaches and pain in their facial muscles, but Natsume thinks there's more to it.
The constant suppression of all unpleasant emotions - and sometimes of all emotions entirely, since the smiles are not supposed to alter with positive emotions any more than with negative ones - can cause them to bubble up at inappropriate times. Natsume believes that they can also cause deep depression, social isolation, and estrangement from emotion. A fixed smile can literally impose mental illness. Smile-mask syndrome, as Natsume calls it, seems to be limited to Japan, but there have been cases reported in Korea. So, if you want to be healthy, turn that frown upside down. And then upside down again.