I'm halfway through a book that is a collection of studies on how people react to failed prophecy; the book is called "Expecting Armageddon, Essential Readings in Failed Prophecy" edited by Jon R. Stone.
Article after article describes how a person can accept the most outrageous explanation, even when all evidence points to the contrary. One of the factors that helps a person accept unreasonable beliefs, is that he is surrounded others who believe it too. You might have heard of "groupthink". (Do follow this video link; it is hilarious)
It seems to me that we have some base instincts that drive us to follow the crowd. This would be a useful evolutionary tactic for creatures who live in packs; including humans. Once I identify with my "crowd", I will fiercly defend it. In return, I get protection by that "crowd". This ability to create groups with strong identities, even to the point of outrageous reason, may be called "granfalloons". What a grand word.
Besides the unreasonableness of some of these groups, I think another great hazard is the suppression of individuality. We naturally give up some of our rights to the crowd, in the interests of mutual safety. I suggest, though, that if the group insists in it's own identity being protected at the cost of individual, it has gone too far.
We see this happen in some groups (think high school) where those who are too different, and the dissenters, are shunned, mocked, or worse.
In Western society, where we worship individualism and tend to dismiss the power of groups over us, I think we underestimate the terrible power of expulsion from a group. The sense of loneliness and abandonment is severe.
What is the balance? Individuality must be protected, but extreme individuality is anarchy. The interests of the group are valuable, but a group that insists that it's own values and maintained at the sacrifice of the individual, suppresses.
The author who expressed this struggle best that I've seen, is Jean Vanier and his book Becoming Human. Jean Vanier is the founder of L'Arche, where people are invited to partner and live with people with mental handicaps, in order to get in touch with the essence of humanness. The commitment of these pairings is intense. Vanier's vision of a healthy group is one with soft borders, that has a shared vision of inclusion rather than exclusion. Healthy group, healthy individuals.