Communal reinforcement is a social phenomenon in which a concept or idea is repeatedly asserted in a community, regardless of whether sufficient empirical evidence has been presented to support it.
Over time, the concept or idea is reinforced to become a strong belief in many people's minds, and may be regarded by the members of the community as fact.
Often, the concept or idea may be further reinforced by publications in the mass media, books, or other means of communication.
The phrase "millions of people can't all be wrong" is indicative of the common tendency to accept a communally reinforced idea without question, which often aids in the widespread acceptance of urban legends, myths, and rumors.
Communal reinforcement works both for true and false concepts or ideas, making the communal reinforcement of an idea independent of its truth value.
Therefore, the fact that many people in a given community believe a certain thing is not indicative of its truth or falsehood, for just as a false concept or idea can be accepted as fact in a community via communal reinforcement, so can a true concept or idea.
Communal reinforcement can be seen as a positive force in society if it reinforces a concept or idea which is true or beneficial to society, such as the discouragement of driving under the influence. Conversely, it can be seen as a negative force if it reinforces a concept or idea which is untrue or harmful to society, such as the avoidance of bathing in Medieval Europe. ~Skeptic's Dictionary