Groupthink is a very well-documented source of bias in cognitive psychology. It refers to the tendency of humans to tend to agree with each other, and hold back objections or dissent even when the group is clearly moving in a very wrong direction.
There seems to be a balance of tensions between evaporative cooling of beliefs leading to groupthink, and extremely biased color politics. This is an underexplored issue.
In computer-mediated deliberation, the ideology of so-called "virtual community" has been implicated as an extremely damaging source of groupthink. In the real world, a "community" is an ethical and political compromise of values due to a shared need for protection from bodily harm or harm to one's surrounding environment. A "virtual community" has no such natural values but still retains the mind-killing illusion of protection, and is thus liable to turn into a pointless cult.