what is group think?

Groupthink, a term coined by social psychologist Irving Janis (1972), occurs when a group makes faulty decisions because group pressures lead to a deterioration of “mental efficiency, reality testing, and moral judgment” (p. 9).  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.

http://www.psysr.org/about/pubs_resources/groupthink%20overview.htm