Smiling may be an indicator of other things besides happiness. It's been proposed (see e.g. http://www.epjournal.net/wp-content/uploads/EP10371397.pdf http://ww

Smiling may be an indicator of other things besides happiness. It's been proposed (see e.g. http://www.epjournal.net/wp-content/uploads/EP10371397.pdf http://www.epjournal.net/wp-content/uploads/EP10371397.pdf though I make no claim that that particular article proves it) that one function of smiling is an acknowledgement of lower social status than the person one's smiling at. If something like this is right, then rich people might smile less at any given level of happiness. (Or: they may smile less in public, which will have a large influence on your perception of rich people's smiling if you don't happen to spend a lot of time in personal interactions with rich people. Or: they may in fact be less happy and therefore smile less -- though the evidence seems to be against this.)