For instance, one supplemental explanation for the False Consensus Effect (because just because it is one effect doesn't mean it has only one cause) that I have heard is that in most cases it is a "free" way of obtaining comfort.

If presented with an opportunity to believe that other people are like you, with no penalty for being wrong, one could expect people will err on the side of predicting behavior consistent with one's own behavior.

I obviously haven't done this experiment, but I suspect that if the subjects asked to wear the sign were offered a cash incentive based on their accuracy of prediction for others, both groups would make a more accurate prediction.

[See also - political predictions are more accurate when the masses are asked to make monetary bets on the winner of the election, rather than simply indicate who they would vote for]