The important thing about groupthink is that it works not so much by censoring dissent as by making dissent seem somehow improbable. As the historian Arthur Schlesinger Jr. put it, “Our meetings took place in a curious atmosphere of assumed consensus.” Even if at first no consensus exists – only the appearance of one – the group’s sense of cohesiveness works to turn the appearance into reality, and in doing so helps dissolve whatever doubts members of the group might have. This process obviously works all the more powerfully in situations where the group’s members already share a common mind-set. Because information that might represent a challenge to the conventual wisdom is either excluded or rationalized as obviously mistaken, people come away from discussions with their beliefs reinforced, convinced more than ever that they’re right. Deliberation in a groupthink setting has the disturbing effect not of opening people’s minds but of closing them.
James Surowiecki, The Wisdom of Crowds, 37-38.