For a study published in 2015, an international group of researchers looked at the number of “source countries” that have fed into various nations since the year 1500. Places like Canada and the United States are very diverse, with 63 and 83 source countries, respectively, while countries like China and Zimbabwe are fairly homogenous, with just a few nationalities represented in their populations.That makes plenty of sense, especially since so many non-immigrant Americans only speak one language. It may also be because we spend so much money on our teeth, we want to show them off. Other reasons for the American smile are explored in an article at the Atlantic.
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.