Find some of Olga Khazan's articles on the subject (The Atlantic) :
Why Americans Smile So Much
"It turns out that countries with lots of immigration have historically relied more on nonverbal communication. Thus, people there might smile more."
"After polling people from 32 countries to learn how much they felt various feelings should be expressed openly, the authors found that emotional expressiveness was correlated with diversity. In other words, when there are a lot of immigrants around, you might have to smile more to build trust and cooperation, since you don’t all speak the same language."
Why Some Cultures Frown on Smiling
"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, a psychologist at the Polish Academy of Sciences. 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."