The Atlantic summarizes a recent study:
Why do some societies not encourage casual smiling? I got my answer, or at least part of one, when I stumbled across a new paper by Kuba Krys [Kuba Krys? Didn't he lay down a smokin' freestyle on that Kendrick Lamar album? - ed.], a psychologist at the Polish Academy of Sciences [Oh, that Kuba Krys - ed.]. In some countries, smiling might not be a sign of warmth or even respect. It’s evidence that you’re a fool—a tricky fool.
Krys focused on a cultural phenomenon called “uncertainty avoidance.” Cultures that are low on this scale tend to have social systems—courts, health-care systems, safety nets, and so forth—that are unstable. Therefore, people there view the future as unpredictable and uncontrollable.
Smiling is a sign of certainty and confidence, so when people in those countries smile, they might seem odd. Why would you smile when fate is an invisible wolf waiting to shred you? You might, in those “low-UA” countries, even be considered stupid for smiling.
Krys also hypothesized that smiling in corrupt countries would be, um, frowned upon. When everyone’s trying to pull one over on each other, you don’t know if someone’s smiling with good intentions, or because they’re trying to trick you....
He found that in countries like Germany, Switzerland, China, and Malaysia, smiling faces were rated as significantly more intelligent than non-smiling people. But in Japan, India, Iran, South Korea, and—you guessed it—Russia, the smiling faces were considered significantly less intelligent. Even after controlling for other factors, like the economy, there was a strong correlation between how unpredictable a society was and the likelihood they would consider smiling unintelligent.
In countries such as India, Argentina, and the Maldives, meanwhile, smiling was associated with dishonesty—something Krys found to be correlated to their corruption rankings.
I've lived here in Tschermany long enough to witness a change or two. One is the increase in smiles on websites. Just anecdotally, I think the percentage of people smiling in websites about firms and universities has risen steadily. At one point, smiling too much in German would get you the reputation of being 'unseriös', but that seems to be fading these days. Also, cosmetic dentistry is becoming mainstream and affordable here. With the standard delay -- about 20 years after this happened in the USA.