Groupthink

Definition: Groupthink is a mode of thinking people engage in when concurrence-seeking (maintaining group cohesiveness, solidarity and unanimity) becomes so dominant in a cohesive ingroup that it tends to override realistic appraisal of alternative courses of action.
The main principle of G is that the more amiability and esprit de corps there is among the members of a policy-making ingroup, the greater the danger that independent critical thinking will be replaced by G, which is likely to result in irrational and dehumanizing actions directed against outgroups.

More on groupthink. More on group decision making: Delphi Method, Metaplan, Spiral of Silence.