What a Russian Smile Means

Alternative hypothesis (but not necessarily mutually exclusive) is that more diverse societies use smiling as a form of non-verbal communication and social bonding.

See, e.g., https://www.theatlantic.com/science/archive/2017/05/why-amer...

The above discusses immigration and language difficulties, but I grew up in the Deep South (where blacks and whites live in much closer proximity than in the North, but in segregated social worlds) and it makes more sense to me that smiling is less about overcoming spoken language hurdles and more about coming across as non-threatening in social contexts where people are naturally uncertain of each other's disposition. I think it's why Southerners emphasize pleasantries so much--as a mechanism to reinforce normative behavior and, specifically, minimize hostilities. Thus, in the U.S. and especially in the Deep South lack of smiling implies hostility, particularly if there's a racial or class divide. It's less obvious today but in more violent times a missing smile was often quite deliberate and strongly communicative.

I have a suspicion that a more common and more cross-cultural non-communicative signal is the head nod. Most women are oblivious to this signal but among men (certainly American men) it seems almost instinctual. And I'm pretty sure it operates as a non-hostility signal. It's less common today but if you try it you'll be surprised at how quickly and immediate the response is; surprising precisely because it's so uncommon. Most women I've tried it with were oblivious or unresponsive, but men almost universally respond in kind, smile, or otherwise respond (e.g. quickly lock eyes). I've tried it in a few other countries and gotten similar responses.

Women should try it out, too; I think most men would respond similarly, though there's always the risk a male might perceive something sexual. I think as we become a more gender equal society it would be useful for women to adopt some of the mechanisms men have used to navigate the social world, just as we encourage men to adopt some of women's tools (e.g. more active listening). A strong society requires strong social bonds. Empathy within a large, diverse society doesn't just magically happen; and without empathy civilization degenerates.