"If all your friends jumped off a bridge, would you jump too?"

"Oh jeez. Probably."

"What!? Why!?"

"Because all my friends did. Think about it -- which scenario is more likely: every single person I know, many of them levelheaded and afraid of heights, abruptly went crazy at exactly the same time... ...or the bridge is on fire?"

Randall Munroe, on updating on other people's beliefs.