When a Reddit user asked, “What’s a dead giveaway that someone is American?” many responses listed one particular trait: wide, enthusiastic smiles. A study suggests that the reason Americans smile so much may have something to do with our immigrant past. In this study, a group of international researchers examined the number of “source countries”—where individuals have emigrated from since the year 1500—in various countries. Canada and the US, for instance, are very diverse, with sixty-three and eighty-three source countries, respectively, while countries such as China and Zimbabwe have only a few different nationalities in their populations. After polling individuals from thirty-two 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, in places with a lot of immigrants from different countries all speaking different languages, you might have to smile more to build friendship, trust, and cooperation.