Groupthink is a term first coined by William H. Whyte in 1952 as:

Groupthink being a coinage — and, admittedly, a loaded one — a working definition is in order. We are not talking about mere instinctive conformity — it is, after all, a perennial failing of mankind. What we are talking about is a rationalized conformity — an open, articulate philosophy which holds that group values are not only expedient but right and good as well.

Irving Janis, who did extensive work on the subject, defined it as:

A mode of thinking that people engage in when they are deeply involved in a cohesive in-group, when the members' strivings for unanimity override their motivation to realistically appraise alternative courses of action.

In other words, it is a situation when a group becomes so tight and together that members of the group are willing to overlook flaws which the group may have for group unity.

The symptoms of groupthink are:
- Illusion of invulnerability
- Unquestioned belief in group's morality
- Stereotyping anyone outside the group as inferior
- Direct pressure to conform placed on any member who questions the group
- Self censorship of ideas that deviate from the apparent group consensus
- Illusions of unanimity among group members
- Mindguards which are self-appointed members who shield the group from dissenting information

In this modern era, some argue that religion is a form of groupthink at its worst. Of course, this is not true for all religions. The most explicit form of religious extremism is usually found in religions worshiping the God of the Desert.

Let's go over the points
- Illusion of invulnerability
The classical belief that one's 'soul' shall live on after death and that their god is watching over them. May lead to excessive optimism and recklessness which may cause more harm than good.

- Unquestioned belief in group's morality
The view of "If it's written in our holy scripture, it has to be true!". May cause delusional thinking of self-righteousness and self-justification of actions. Has been true for many terrorist attacks and the crusades.

- Stereotyping anyone outside the group as inferior
The view of "Everyone else is wrong, we are the ones who are right". This is mostly true for evangelists who preach about the gospel to perfectly happy and decent people just because they are of a different religion.

- Direct pressure to conform placed on any member who questions the group
Threats that deal with the member who questions the group usually ending up in alone and defenseless. This is quite similar to peer pressure in its workings.

- Self censorship of ideas that deviate from the apparent group consensus
This is usually true for the members who are in the grey area between "all for" and "not for" the group. Inner conflicts arise in those members and may cause mental stress.

- Illusions of unanimity among group members
This one should be pretty obvious. Group members make assumptions that the rest of the group is thinking of the same thing. Also, closure is applied here.

- Mindguards
The ones who keep the group in line with their filtering of dissenting information.

In conclusion, groups should have a regular evaluation and feedback session done by either a specially elected member of the group or a party outside of the group to prevent groupthink. Critical examination of all alternative options should also be examined and considered.