False consensus

False consensus or the effect of the false consensus is the tendency for people to assume that everyone else thinks the same way they do.
It is a cognitive bias in which a person overestimates the "agreement grade" that the other people have with them. In other words, people tend to think that others agree with them. Most of the time, a person thinks that their own attitudes, beliefs, values and habits are the most common ones. In reality, the person's thoughts may not be shared by others. 
Another effect which can occur with false consensus is called pluralistic ignorance. In pluralistic ignorance, people support a norm or a belief in public, even though they really think it is wrong.