Groupthink is based on the idea that as group members try to minimize conflict, they reach a consensus without critically testing, analyzing, and evaluating ideas.  As described by Irving Janis, it is a "deterioration of mental efficiency, reality testing and more judgment which results from in-group pressure" (Beck et al., 2005).   This is a direct result of conhesiveness in groups, or the degree of mutual interest among members.  Therefore, in an ideal highly conhesive group, a strong mutual idenitification helps keep it together.  Cohesivness can be a good thing as it brings people together and strengthens the group's interpersonal relationships; yet Janis also reconigzes the dangers in it as he notes that highly cohesive groups can harm the progress of a group if too much attention and energy is spent focusing on maintaining goodwill and establishing positive bonds (Littlejohn, 2002). 

Symptoms

From his research, Janis discovered a list of symptoms that can function in the diagnosis of groupthink:
1.  Illusion of invulnerability
2.  Collective rationalism
3.  Belief in inherent morality of group
4.  Stereotype of out-groups
5.  Direct pressure on dissenters
6.  Self-censorship
7.  Illusion of unanimity
8.  Self-appointed mind-guard

(Beck et al., 2005, 233)