Social scientists such as Szlemko say that people carry around three kinds of territorial spaces in their heads. One is personal territory -- like a home, or a bedroom. The second kind involves space that is temporarily yours -- an office cubicle or a gym locker. The third kind is public territory: park benches, walking trails -- and roads.
Increased territoriality leads people to treat public and temporary territory the same as personal territory. So as the public becomes the personal, it seems so would more abstract public notions become personal--the sense of ownership and territoriality would extend to ideas, which would bring about frustration when different or opposing ideas came into that extended personal space. Read my bumper sticker, love my stance, as it were.
This is something I'd considered, so I'm glad to see the validation. The article also notes that the more bumper stickers a car has, the more aggressive the driver (where owner and stick-er are the same person). What strikes me about people with lots of bumper stickers is the noise level of their expression--so much to say, and making damn sure that it gets broadcast. I think this goes equally for t-shirts, buttons, tote bags, whatever.
More on this as I think about it.