Bob Rowan / Progressive Image-Corbis 

I used to think that the most dangerous thing about bumper stickers was that they make curious drivers inch ever closer to the car in front of them in order to read the things (“He Put the Duh in W,” perhaps, or “At Least the War on the Environment is Going Well,” or “49% ***, 51% Sweetheart; Don’t Push It,” or “If There Is a Tourist Season, Why Can’t We Shoot Them?”—for all of which I am indebted to http://www.bumperart.com). But no, bumper stickers pose another danger: drivers who plaster their vehicles with the things are more prone to road rage than drivers who leave their car or truck unadorned.

As scientists led by Paul Bell, Lucy Troup and Bell's student William Szlemko of Colorado State University report in the June issue of the Journal of Applied Social Psychology, it’s a simple matter of territoriality. Researchers have long known that drivers who have a strong sense of personal space while in their vehicle are more likely to be road-ragers, and the more someone plasters his vehicle with bumper stickers and decals the more territorial he feels about the space inside.

So why are you making that face? Because back when humans were evolving into what we are now, those specific contortions of eyes, nose, mouth and cheeks were adaptive—that is, useful for surviving.

Have you ever wondered why people the world over make essentially the same face when they’re afraid, whether they’re Maori tribesmen facing a stampede or Wall Street titans being handed a subpoena from the SEC? One explanation for that sameness has been that the expressions arose randomly, but got locked into human nature because other people could understand them: that is, if everyone made the same face for “I’m afraid,” then others in their group could read and understand it instantly—very useful for times before spoken language or when you can’t hear what the other guy might be shouting.

The same reasoning applies to facial expressions of disgust. If your dinner companion makes a face as he tears into his serving of roast boar, it’s helpful to understand it instantly before you, too, chow down on spoiled meat. The sameness of facial expressions is called cultural invariance, and it means that the Maori tribesman can read the hedge fund manager’s expression and the manager can read the tribesman’s.