Here are two excerpted quotations, cited by John Tierney in his sadly gated blog:

“While we typically think of a smile as displaying our emotional state (happiness), it also appears that smiles convey information about the signaler’s status. Specifically, lower status individuals appear to smile more than higher status individuals. I suspect that this is due, in part, to the fact that there are several different types of smiles, including a true happiness smile and a true embarrassment smile. The latter smile, the embarrassment display, is often seen as an appeasement display in primates. Jimmy Carter smiled a lot, George Bush smiles much less. Jimmy Carter is generally perceived to be warm and friendly, but not very dominant and strong. George Bush is perceived be be somewhat less warm and friendly, but is seen as quite dominant and strong.”

And: