The false-consensus effect is a cognitive bias that was coined by researcher Lee Ross and his colleagues in 1976. In one experiment, study participants had to decide which of two possible ways they'd respond to a specific conflict, such as whether they'd contest a ticket for which they may be guilty, or whether to vote for a space-research funding bill that would lead to higher taxes. They were also asked which option they believed other people would choose, and to describe the type of people who would make those choices. In the end, regardless of which option the participants chose, they tended to think that most people would choose the same one. Interestingly, the participants also gave more extreme descriptions of the people who didn't choose their preferred option. They viewed their own choice as normal, thus assuming that the majority of people would make the same decision.
But why? The previous pizza scenario highlights one possible cause of the false-consensus effect called the availability heuristic. While estimating the likelihood that others share your beliefs, you'll probably consider the first people who come to mind: your family and friends. The fact that so many examples of people agreeing with you come to mind so quickly might lead you to believe that a higher proportion of the population loves briney pizza toppings than actually does. The false-consensus effect might also be a way to preserve self-esteem. We're motivated to think that other people share our thoughts and feelings—if they didn't, that might suggest there's something wrong with us.
When do people usually fall prey to the false-consensus effect? The more invested and confident we are about something, the more we assume others agree with us. To learn more about cognitive biases, view the following videos.