en.wikipedia.org/wiki/Groupthink
Groupthink is a psychological phenomenon that occurs within a group of people in which the desire for harmony or conformity in the group results in an irrational or dysfunctional decision-making outcome. Group members try to minimize conflict and reach a consensus decision without critical evaluation of alternative viewpoints by actively suppressing dissenting viewpoints, and by isolating themselves from outside influences.
Groupthink requires individuals to avoid raising controversial issues or alternative solutions, and there is loss of individual creativity, uniqueness and independent thinking. The dysfunctional group dynamics of the "ingroup" produces an "illusion of invulnerability" (an inflated certainty that the right decision has been made). Thus the "ingroup" significantly overrates its own abilities in decision-making and significantly underrates the abilities of its opponents (the "outgroup"). Furthermore, groupthink can produce dehumanizing actions against the "outgroup".
To make groupthink testable, Irving Janis devised eight symptoms indicative of groupthink.
Type I: Overestimations of the group — its power and morality
Illusions of invulnerability creating excessive optimism and encouraging risk taking.
Unquestioned belief in the morality of the group, causing members to ignore the consequences of their actions.
Type II: Closed-mindedness
Rationalizing warnings that might challenge the group's assumptions.
Stereotyping those who are opposed to the group as weak, evil, biased, spiteful, impotent, or stupid.
Type III: Pressures toward uniformity
Self-censorship of ideas that deviate from the apparent group consensus.
Illusions of unanimity among group members, silence is viewed as agreement.
Direct pressure to conform placed on any member who questions the group, couched in terms of "disloyalty"
Mindguards— self-appointed members who shield the group from dissenting information.