Here is yet another important consideration for
helping us to understand the individual in a group:
Moreover, by the mere fact that he forms part of
an organised group, a man descends several rungs
in the ladder of civilization. Isolated, he may be a
cultivated individual; in a crowd, he is a barbarian — that is, a creature acting by instinct. He possesses
the spontaneity, the violence, the ferocity, and also
the enthusiasm and heroism of primitive beings.
He then dwells especially upon the lowering in
intellectual ability which an individual experiences when
he becomes merged in a group.

A group experience takes place on a lower level of consciousness than the experience of an individual. This is due to the fact that, when many people gather together to share one common emotion, the total psyche emerging from the group is below the level of the individual psyche. If it is a very large group, the collective psyche will be more like the psyche of an animal, which is the reason why the ethical attitude of large organizations is always doubtful. The psychology of a large crowd inevitably sinks to the level of mob psychology. If, therefore, I have a so-called collective experience as a member of a group, it takes place on a lower level of consciousness than if I had the experience by myself alone.

You know how it is when you go to be the subject of a psychology experiment and nobody else shows up and you think maybe that's part of the experiment? I'm like that all the time.

Assorted theories have been advanced to explain confirmation bias — why people rush to embrace information that supports their beliefs while rejecting information that disputes them: that first impressions are difficult to dislodge, that there’s a primitive instinct to defend one’s turf, that people tend to have emotional rather than intellectual responses to being challenged and are loath to carefully examine evidence.
Group dynamics only exaggerate these tendencies, the author and legal scholar Cass Sunstein observed in his book Going to Extremes: insularity often means limited information input (and usually information that reinforces preexisting views) and a desire for peer approval; and if the group’s leader “does not encourage dissent and is inclined to an identifiable conclusion, it is highly likely that the group as a whole will move toward that conclusion.”
Once the group has been psychologically walled off, Sunstein wrote, “the information and views of those outside the group can be discredited, and hence nothing will disturb the process of polarization as group members continue to talk.” In fact, groups of like-minded people can become breeding grounds for extreme movements. “Terrorists are made, not born,” Sunstein observed, “and terrorist networks often operate in just this way. As a result, they can move otherwise ordinary people to violent acts.

I knew that, in a large degree, we were trying an experiment — that of testing whether or not it was possible for Negroes to build up and control the affairs of a large education institution. I knew that if we failed it wold injure the whole race.

In essence, we tend to sort each other and ourselves into groupings, and that usually leads to an overestimation of similarities between members of a group, and an underestimation of the similarities between insiders and outsiders.

Put any company of people together with freedom for conversation, and a rapid self-distribution takes place into sets and pairs. The best are accused of exclusiveness. It would be more true to say they separate as oil from water, as children from old people, without love or hatred in the matter, each seeking his like; and any interference with the affinities would produce constraint and suffocation. All conversation is a magnetic experiment, I know that my friend can talk eloquently; you know that he cannot articulate a sentence: we have seen him in different company.

I have performed the following experiment in workshops for nearly 40 years now: Everybody in the class is asked to describe the hall they passed through to get to the classroom. I must have tried this several hundred times by now, and I have never encountered two people who agreed totally about what was or was not in the hall, the color of the walls, or any similar data. We do not walk through the “same” hall: we walk through a reality-tunnel constructed by our imprinted, conditioned and learned brain circuits.

The same experiment works with hearing, and other senses, as well as with vision and memory. Try it with a half-dozen friends. Let somebody with a watch say “Go!” and then all of you be silent and listen for one full minute — 60 surprisingly long seconds. You will all hear some sounds nobody else hears and miss some sounds everybody else caught.

From 'In Doubt We Trust: Cults, religions, and BS in general

"The "us versus them" mindset coupled with our social nature implies that we have an innate need to belong to clearly defined in-groups."

Alas, one cannot assert authority by accepting one's own fallibility. Simply, people need to be blinded by knowledge-we are made to follow leaders who can gather people together because the advantages of being in groups trump the disadvantages of being alone. It has been more profitable for us to bind together in the wrong direction than to be alone in the right one. Those who have followed the assertive idiot rather than the introspective wise person have passed us some of their genes. This is apparent from a social pathology: psychopaths rally followers.

The larger the group, the more toxic, the more of your beauty as an individual you have to surrender for the sake of group thought. And when you suspend your individual beauty you also give up a lot of your humanity. You will do things in the name of a group that you would never do on your own. Injuring, hurting, killing, drinking are all part of it, because you've lost your identity, because you now owe your allegiance to this thing that's bigger than you are and that controls you.

and still am, suspicious of the idea of oneness. I don’t buy into the homogeneity of the human experience. I don’t think we’re all one. We can be one, but I don’t think we have to see things the same way for that to be so. An anarchic thought: We’re one but we’re not the same. We get to carry each other, not that we’ve got to, just that we get to.

But when the group is literally capable of changing our perceptions, and when to stand alone is to activate primitive, powerful, and unconscious feelings of rejection, then the health of these institutions seems far more vulnerable than we think.

Every faction conditions its members to think and act a certain way. And most people do it. For most people, it's not hard to learn, to find a pattern of thought that works and stay that way. But our minds move in a dozen different directions. We can't be confined to one way of thinking, and that terrifies our leaders. It means we can't be controlled. And it means that no matter what they do, we will always cause trouble for them.