> That surprised me, since our cultural norms largely stemmed from [England].

I don't think that assumption really holds up, it's very pop-history. From the Scots of the Appalachians, the religious fanatics of New England, the garguntuan influence of African-American syncretic culture, the Nordic yeoman of the mid-north-east, the southern European urban influx of the 1900s and the new, exciting Latin American syncretism: America really is a cultural melting pot. Only, really, the Virginia gentry (Jefferson, Washington, et al) can be plainly said to have imported English norms - and still, they were ideological radicals interested in forming a new nation.

The French like to call us (English, Scots, and all the varieties of American) "Anglo-Saxons," but they're hardly right. Don't give them ammo, they're already merciless!

There's a paper floating around somewhere that finds a positive correlation between polite smiling and diversity. It posits that it's a way to help establish trust in societies where you're constantly interacting w/ people from groups outside your personal sphere. Indeed, I suspect Americans probably smile even more when abroad precisely because they're interacting w/ social strangers.

May be inherited from the cultures that intermingled in the Americas? Africans tend to smile a lot.