False consensus effect

The false consensus effect refers to the tendency for people to overestimate the degree to which others agree with them. People readily guess their own opinions, beliefs and predilections as being more prevalent in the general public than they really are.

This bias is commonly present in a group setting where one thinks the collective opinion of their own group matches that of the larger population. Since the members of a group reach a consensus and rarely encounter those who dispute it, they tend to believe that everybody thinks the same way.

There is no single cause for this cognitive bias; the availability heuristic and self-serving bias have been suggested as at least partial underlying factors.