In psychology, the false consensus effect, also known as consensus bias, is a pervasive cognitive bias that causes people to overestimate the extent to which other people share their beliefs and views; it is the tendency to "see their own behavioral choices and judgments as relatively common and appropriate to existing circumstances". In other words, they assume that their personal qualities, characteristics, beliefs, and actions are relatively widespread through the general population.

This false consensus is significant because it increases self-esteem (overconfidence effect). 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. 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.

The false consensus effect has been widely observed and supported by empirical evidence. One recent study has shown that consensus bias may improve decisions about other people's preferences. Ross, Green and House first defined the false consensus effect in 1977 with emphasis on the relative commonness that people perceive about their own responses; however, similar projection phenomena had already caught attention in psychology. Specifically, concerns with respect to connections between individual's personal predispositions and their estimates of peers appeared in the literature for a while. For instances, Katz and Allport in 1931 illustrated that students’ estimates of the frequency of cheating by others was positively correlated to their own behavior. Later, around 1970, the same phenomena were found on political beliefs and prisoner's dilemma situation. In 2017, researchers identified a persistent egocentric bias when participants learned about other people's snack-food preferences. Moreover, recent studies suggest that the false consensus effect can also affect professional decision makers; specifically, it has been shown that even experienced marketing managers project their personal product preferences onto consumers.

There is no single cause for this cognitive bias; however, several underlying mechanisms have been suggested to contribute to its formation and maintenance. Previous research has suggested that cognitive and perceptional factors (motivated projection, accessibility of information, emotion, etc.) may contribute to the consensus bias, while recent studies have focused on its neural mechanisms. The bias may also result, at least in part, from non-social stimulus-reward associations.

Cognitive mechanisms, such as the availability heuristic, self-serving bias, and naïve realism have been suggested as at least partial underlying factors in the False Consensus Effect. The availability heuristic is a mental shortcut that people default to, in which people may incorrectly attribute the likelihood or commonness of something based on how cognitively available the concept is to them, or how quickly it comes to mind; this could contribute to the False Consensus Effect when individuals have a readily available concept, causing them to overestimate its commonality. Self-serving bias is an attribution error that describes the tendency to attribute successes and positive traits to one's own internal factors, and attribute failures or negative traits to the external environment. This can contribute to the False Consensus Effect by justifying our actions with self-serving bias, and consequently using the False Consensus Effect to reinforce that those actions were acceptable by believing our views are widely shared. Naïve realism is the idealist belief that we perceive the world accurately, and individuals who disagree with our perceptions are incorrect or biased; this contributes to the False Consensus Effect by reinforcing that people who disagree with our view are part of the minority, whereas the majority still agrees with us.

The False Consensus Effect can be partially attributed to the innate desire to conform and be liked by others in a social environment by sharing characteristics with members of a social group, within the parameters determined by the social environment; these parameters can be influenced by demographic factors, such as age, gender, and socioeconomic status, and cultural differences. The innate motivation to be liked is known as normative social influence, conceptualized by revolutionary social psychologist Solomon Asch in 1951. Normative social influence is a social and evolutionary function to share characteristics with a group, form a group identity, and benefit from the protection and resources of group membership. It can cause the False Consensus Effect by creating a social illusion - the need to be liked causes one to agree with others outwardly even if they disagree internally, creating a social illusion of collective agreement. Additionally, the False Consensus Effect is fundamentally a perceptual effect; normative social influence motivates individuals to agree with each other, potentially leading some to believe that everyone getting along socially means that everyone agrees. Normative social influence also leads to people feeling validated in their beliefs when they are not challenged, reinforcing the illusion of correctness and group cohesion.

Another type of social pressure to conform is informational social influence, also coined by Asch, that may contribute to the False Consensus Effect. This describes individuals' tendency to conform to a majority consensus out of the need to be correct; additionally, Asch posited that informational social influence is partially caused by people learning how to act within socially determined guidelines by perceiving others' behavior, allowing them to fall into the cohesive group identity. Maintenance of the False Consensus Effect 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.

The false-consensus effect can be traced back to two parallel theories of social perception, "the study of how we form impressions of and make inferences about other people". The first is the idea of social comparison. The principal claim of Leon Festinger's (1954) social comparison theory was that individuals evaluate their thoughts and attitudes based on other people. This may be motivated by a desire for confirmation and the need to feel good about oneself. Informational social influence can be viewed as an extension of this theory, where people may use others as sources of information to define social reality and guide behavior. This is called informational social influence. The problem, though, is that people are often unable to accurately perceive the social norm and the actual attitudes of others. In other words, research has shown that people are surprisingly poor "intuitive psychologists" and that our social judgments are often inaccurate. This finding helped to lay the groundwork for an understanding of biased processing and inaccurate social perception. The false-consensus effect is just one example of such an inaccuracy.