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".

Antecedent factors such as group cohesiveness, faulty group structure, and situational context (e.g., community panic) play into the likelihood of whether or not groupthink will impact the decision-making process.

Groupthink is a construct of social psychology, but has an extensive reach and influences literature in the fields of communication studies, political science, management, and organizational theory,[1] as well as important aspects of deviant religious cult behaviour.[2][3]