WHY DO AMERICANS SMILE SO MUCH? Because America is diverse, and in diverse societies people need to signal amiability. “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. People in the more diverse countries also smiled for a different reason than the people in the more homogeneous nations. In the countries with more immigrants, people smiled in order to bond socially.”

Hmm. Okay, this may just be bunk, of course. But if it’s true it has implications that the story ignores: Are more-diverse societies inherently more tense? Do people feel less stress in the presence of people with whom they share some vague genetic connection?