“Groups have powerful self-reinforcing mechanisms at work. These can lead to group polarization—a tendency for members of the group to end up in a more extreme position than they started in because they have heard the views repeated frequently.
At the extreme limit of group behavior is groupthink. This occurs when a group makes faulty decisions because group pressures lead to a deterioration of “mental efficiency, reality testing, and moral judgment.” The original work was conducted with reference to the Vietnam War and the Bay of Pigs fiasco. However, it rears its head again and again, whether it is in connection with the Challenger space shuttle disaster or the CIA intelligence failure over the WMD of Saddam Hussein.

Groupthink tends to have eight symptoms:
1 . An illusion of invulnerability. This creates excessive optimism that encourages taking extreme risks. [...]
2. Collective rationalization. Members of the group discount warnings and do not reconsider their assumptions. [...]
3. Belief in inherent morality. Members believe in the rightness of their cause and therefore ignore the ethical or moral consequences of their decisions.
4. Stereotyped views of out-groups. Negative views of “enemy” make effective responses to conflict seem unnecessary. Remember how those who wouldn't go along with the dot-com bubble were dismissed as simply not getting it.
5. Direct pressure on dissenters. Members are under pressure not to express arguments against any of the group’s views.
6. Self-censorship. Doubts and deviations from the perceived group consensus are not expressed.
7. Illusion of unanimity. The majority view and judgments are assumed to be unanimous.
8. "Mind guards" are appointed. Members protect the group and the leader from information that is problematic or contradictory to the group's cohesiveness, view, and/or decisions. This is confirmatory bias writ large.”