In principle, a cohesive group—one whose members generally agree with one another and support one another's judgments—can do a much better job at decision making than it could if it were noncohesive. When cohesiveness is low or lacking entirely, compliance out of fear of recrimination is likely to be strongest. To overcome this fear, participants in the group's deliberations need to be confident that they are members in good standing and that the others will continue to value their role in the group, whether or not they agree about a particular issue under discussion. As members of a group feel more accepted by the others, they acquire greater freedom to say what they really think, becoming less likely to use deceitful arguments or to play it safe by dancing around the issues with vapid or conventional comments. Typically, then, the more cohesive a group becomes, the less its members will deliberately censor what they say out of fear of being punished socially for antagonizing their fellow members.

But group cohesiveness can have pitfalls as well: while the members of a highly cohesive group can feel much freer to deviate from the majority, their desire for genuine concurrence on every important issue often inclines them not to use this freedom. In a highly cohesive group of decision makers, the danger is not that individuals will conceal objections they harbor regarding a proposal favored by the majority, but that they will think the proposal is a good one without attempting to carry out a critical scrutiny that could reveal grounds for strong objections. Members may then decide that any misgivings they feel are not worth pursuing—that the benefit of any doubt should be given to the group consensus. In this way, they may fall victim to a syndrome known as "groupthink," which one psychologist concerned with collective decision making has defined as "a deterioration of mental efficiency, reality testing, and moral judgment that results from in-group pressures."

Based on analyses of major fiascoes of international diplomacy and military decision making, researchers have identified groupthink behavior as a recurring pattern that involves several factors: overestimation of the group's power and morality, manifested, for example, in an illusion of invulnerability, which creates excessive optimism; closed-mindedness to warnings of problems and to alternative viewpoints; and unwarranted pressures toward uniformity, including self-censorship with respect to doubts about the group's reasoning and a concomitant shared illusion of unanimity concerning group decisions. Cohesiveness of the decision-making group is an essential antecedent condition for this syndrome but not a sufficient one, so it is important to work toward identifying the additional factors that determine whether group cohesiveness will deteriorate into groupthink or allow for effective decision making.