That’s my interpretation anyway. From the always-great Deliberations:

I love the bumper sticker question in voir dire.  I’ve met lawyers and seen journalists who are surprised by it, or think it’s intrusive, but when you think about it, it’s a no-brainer.  If a juror holds an attitude so strongly that she’ll paste it onto her car, you want to know what that attitude is.

New research suggests you should be interested in something else, too.  It isn’t simply what jurors’ bumper stickers say, it’s whether jurors have bumper stickers at all.   Writing in the June issue of the Journal of Applied Social Psychology, Colorado State University researchers suggest that people with bumper stickers are more likely to be aggressive and angry people, or at least aggressive and angry drivers.

It even applies to people with ‘peaceful’ or otherwise conciliatory bumper stickers. I think that’s a no-brainer as well —  if a person is of the type to broadcast their opinions to complete strangers, even where there’s no dialog in return, you can assume they are also generally assertive, the type of people that will lead (or wreck) a jury.