False-Consensus Effect
False-Consensus Bias
The tendency to overestimate how many other people share your opinions, preferences, and habits — assuming your way of thinking is more "normal" than it actually is.
Origin
Social psychologist Lee Ross and colleagues David Greene and Pamela House at Stanford University first demonstrated the effect in a 1977 paper. In one study, they asked students whether they would walk around campus wearing a sandwich-board sign, then estimate what percentage of peers would make the same choice. Regardless of their own decision, participants believed the majority would do the same — establishing what Ross termed the false-consensus effect.
Everyday Use
You assume everyone finds the same jokes funny, votes the same way, or thinks pineapple on pizza is obviously wrong (or obviously right). Then you're genuinely shocked to learn otherwise. The false-consensus effect is the invisible bubble that makes our own views feel like common sense.