From the Dilbert Blog:

[…] I have a related hypothesis. I believe that political conservatives can be identified by their faces, even after you control for haircuts and eyeglasses. Not all the time, of course, but more than chance. (Another way to say the same thing is that political liberals can be identified by their looks.) Have you ever noticed that?

Duh again. 🙂 Here’s my take on it: hard-core conservatives are afraid of everything different — that’s what makes them conservative. That fear triggers the fight-or-flight response, and habitual expressions leave traces on a person’s face, so hard-core conservatives inevitably bear the traces of being habitually afraid or angry, or at least disapproving. (I don’t know my mother’s political orientation — she’d never tell us — but she’s habitually disapproving of anything different, and her face shows it, so I strongly suspect she’s a political conservative. GoddessJ and I also knew a hard-core conservative family at one point, before they decided that we were too different for them and cut us off, and all but the youngest son definitely had the habitually-angry-or-afraid thing down pat.)

By the same token, I’ve noticed that liberals don’t feel threatened by things that are different, and even when they have no interest in practicing the differences themselves, they tend to want to know something about them. Hard-core liberals (of which I know at least one — c-square, you know him too, and I’ll bet you can identify him with no more clue than that 🙂 ) are habitually interested, and that tends to leave detectable traces on their faces as well.

It wouldn’t work all the time, and it wouldn’t work reliably on more middle-of-the-road people, but in my experience it’s pretty accurate.