People tend to believe that their opinions are common. False consensus describes this phenomenon, as people believe that the consensus opinion (or majority opinion) agrees with their own, regardless of what their opinion is.
There is also the idea of false uniqueness, which describes the misjudgment of one’s similarity to others. For example, people are given a choice between two uncomfortable situations. Then they are asked how much discomfort they expected to experience in the situation they chose. Also, they are asked which experience they believed most people would choose, and how much discomfort most people would experience. Subjects responded that they believed others would choose the same alternative they did, but that other people would not experience as much discomfort as they would. Thus, people’s responses suggest others would have the same opinion as they did (make the same choice), but others would not feel as much as they would (experience as much discomfort).
Interestingly, people believe that the ability that they are best at is not shared by many people. However, the same people also believe that the opinion that is the most important to them is commonly held. Thus, people seem to be saying that they are unique and common simultaneously. Unique in ability, and common in belief.