Freud and his nephew Bernays on Group Behavior and lack of Critical Thinking

If one can design propaganda or psychological operations that bypass the conscious and rational faculties of the individual, targeting instead suppressed emotions and hidden desires, it is possible to move people to adopt beliefs and behaviors without them being aware of the underlying motivations leading them on. Men are very largely actuated by motives which they conceal from themselves. It is evident that the successful propagandist must understand the true motives and not be content to accept the reasons which men give for what they do.
(Propaganda, Edward Bernays)

A group is extraordinary credulous and open to influence, it has no critical faculty.
(Group psychology and the Analysis of the Ego, Sigmund Freud)

In identifying with a group, The individual subordinates self analysis and critical thinking, and a Discerning search for the truth in favor of maintaining group interest in cohesion.

By dividing a population along lines such as race class religion gender or political preference, or in other words into groups naturally prone to clash, the effects of group psychology render rational discourse and debate between individuals in the separate groups extremely unlikely. Each group considers its own standards ultimate and indisputable, intends to dismiss all contrary or different standards as indefensible. (Crystallizing Public Opinion, Edward Bernays)