Ad populum

In argumentation theory, an argumentum ad populum is a fallacious argument which is based on claiming a truth or affirming something is good because many people think so.

If something is believed to be true by a lot of people then it must be true.

A variant is where the probability of truth is assessed by the number of people making the assertion (especially when you can see who is voting for and who is voting against the idea).

When we are uncertain about something, we turn to other people and assume they know what they are doing. We do the same with beliefs. The more other people believe something, the more likely we will be to accept that it is true (especially if we tend towards black and white thinking).