https://en.wikipedia.org/wiki/False-consensus_effect
In psychology, the false-consensus effect or false-consensus bias is a cognitive bias whereby a person tends to overestimate the extent to which their opinions, beliefs, preferences, values, and habits are normal and typical of those of others (i.e., that others also think the same way that they do).[1] This cognitive bias tends to lead to the perception of a consensus that does not exist, a "false consensus".
This false consensus is significant because it increases self-esteem. It is derived from a desire to conform and be liked by others in a social environment. This bias is especially prevalent in group settings 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. The false-consensus effect is not restricted to cases where people believe that their values are shared by the majority, but it still manifests as an overestimate of the extent of their belief. For example, fundamentalists do not necessarily believe that the majority of people share their views, but their estimates of the number of people who share their point of view will tend to exceed the actual number.
Additionally, when confronted with evidence that a consensus does not exist, people often assume that those who do not agree with them are defective in some way.[2] There is no single cause for this cognitive bias; the availability heuristic, self-serving bias, and naïve realism have been suggested as at least partial underlying factors. Maintenance of this cognitive bias may be related to the tendency to make decisions with relatively little information. When faced with uncertainty and a limited sample from which to make decisions, people often "project" themselves onto the situation. When this personal knowledge is used as input to make generalizations, it often results in the false sense of being part of the majority.
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