Groupthink, a term coined by social psychologist Irving Janis, occurs when a group makes faulty decisions because group pressures lead to a deterioration of “mental efficiency, reality testing, and moral judgment”. Groups affected by groupthink ignore alternatives and tend to take irrational actions that dehumanize other groups. A group is especially vulnerable to groupthink when its members are similar in background, when the group is insulated from outside opinions, and when there are no clear rules for decision making.
Eight symptoms of groupthink:
The only way to protect yourself from group think is to deliberately try to understand things from another perspective, and that is a lot of work because you never know what set of assumptions and world views the other may be operating under, possibly they don't know either.
It's a process bound to continuous improvement. We are coming from caves and nomads to getting together in groups, not much thinking initially and now we are going through that phase over the span of thousands of years. We can see our past, we can enjoy our present and we can steer to our future.
Here is a bit from Carl Jung's The Four Archetypes on the dangers of group think.
Human nature has a weak habit of taking gifts for granted; in times of necessity, we demand them as a right instead of making the eﬀort to obtain them ourselves. One sees this, unfortunately, only too plainly in the tendency to demand everything from the State, without reﬂecting that the State consists of those very individuals who make the demands. The logical development of this tendency leads to Communism, where each individual enslaves the community and the latter is represented by a dictator, the slave-owner. All primitive tribes characterized by a communistic order of society also have a chieftain over them with unlimited powers. The Communist State is nothing other than an absolute monarchy in which there are no subjects, but only serfs.
The irony is that those caught up in groupthink don’t know they are engaged in groupthink. If it’s someone else’s group it’s very easy to make that characterization—to view them that way. But remember, from their perspective it might be you who are caught up in groupthink.
Maybe it’s not absolute. Maybe there are elements of groupthink in all of us from time to time, but we also are critical thinkers in some areas.