There are several main causes of groupthink. These include group cohesiveness, overall group isolation, group leadership, and decision-making stress. High levels ofcohesiveness decrease the amount of verbal dissension within a tight group, due to interpersonal pressure to conform. This high level of cohesiveness also creates self-censorship and apparent unanimity within the group. Normally, group dissension is necessary for good decision-making, because it introduce different perspectives to the decision-making process. In the absence of this disagreement, alternative choices for action are never considered. Another cause of groupthink is isolation. Often in group situations, it is important that the decisions being made or the actions being carried remain secret. This requires that no outside opinions or thoughts be incorporated into the decision-making process. Frequently, groups reach resolutions and carry them out without conferring with any outside sources. One result of this extreme isolation is insulation from criticism. This absence of criticism may lead to illusions of group invulnerability and morality. The leadership of a group can also lead to groupthink, since complete control over the group by the leader can cause an environment in which no one states their own opinions. When extremely rigid leadership is implemented within a group (such as occurs in the military) group discussions are often tightly controlled. Any dissenting opinions tend to be suppressed through intimidation or be simply not allowing the dissenter to voice his objections. If a leader in a group situation makes his opinion clear at the outset of the discussions, group members will on many occasions refrain from expressing any disagreement out of respect for the leader’s authority.