http://www.wired.com/wiredscience/2011/05/wisdom-of-crowds-decline/

"In a new study of crowd wisdom — the statistical phenomenon by which individual biases cancel each other out, distilling hundreds or thousands of individual guesses into uncannily accurate average answers — researchers told test participants about their peers’ guesses. As a result, their group insight went awry."

Seems to be a good explanation for how religion can override individual common sense by convincing them that everyone else believes what the religion teaches when in fact there are probably more unbelievers than believers.