@PUNKEY,
I think a lot of the other responses to this question may be more accurate, but I really think it comes down to group-think and social comparison.
For example, on a small scale, someone might hail something as true. This person may be the leader and the group doesn't want to displease them, so they agree.
Then from there, they spread it and look down on other people for not believing the same thing. They may ridicule or make them look a fool, so much so, that they conform.
I think it has nothing to do with truth, but it has everything to do with social interactions.