Prejudice is an affective feeling towards a person or group member based solely on their group membership. The word is often used to refer to preconceived, usually unfavorable, feelings towards people or a person because of their sex, gender, beliefs, values, social class, age, disability, religion, sexuality, race / ethnicity, language, nationality, beauty, occupation, education, criminality, sport team affiliation or other personal characteristics. In this case, it refers to a positive or negative evaluation of another person based on their perceived group membership.
Prejudice can also refer to unfounded beliefs  and it may include "any unreasonable attitude that is unusually resistant to rational influence". Gordon Allport defined prejudice as a "feeling, favorable or unfavorable, toward a person or thing, prior to, or not based on, actual experience". For the evolutionary psychology perspective, see Prejudice from an evolutionary perspective. Auestad (2015) defines prejudice as characterized by 'symbolic transfer', transfer of a value-laden meaning content onto a socially formed category and then on to individuals who are taken to belong to that category, resistance to change, and overgeneralization. 
The first psychological research conducted on prejudice occurred in the 1920s. This research attempted to prove white supremacy. One article from 1925 which reviewed 73 studies on race concluded that the studies seemed "to indicate the mental superiority of the white race".  These studies, along with other research, led many psychologists to view prejudice as a natural response to inferior races.
In the 1930s and 1940s, this perspective began to change due to the increasing concern about anti-Semitism. At the time, theorists viewed prejudice as pathological and they thus looked for personality syndromes linked with racism. Theodor Adorno believed that prejudice stemmed from an authoritarian personality; he believed that people with authoritarian personalities were the most likely to be prejudiced against groups of lower status. He described authoritarians as "rigid thinkers who obeyed authority, saw the world as black and white, and enforced strict adherence to social rules and hierarchies".
In 1954, Gordon Allport linked prejudice to categorical thinking. Allport claimed that prejudice is a natural and normal process for humans. According to him, "The human mind must think with the aid of categories… Once formed, categories are the basis for normal prejudgment. We cannot possibly avoid this process. Orderly living depends upon it."
In the 1970s, research began to show that prejudice tends to be based on favoritism towards one's own groups, rather than negative feelings towards another group. According to Marilyn Brewer, prejudice "may develop not because outgroups are hated, but because positive emotions such as admiration, sympathy, and trust are reserved for the ingroup." 
In 1979, Thomas Pettigrew described the ultimate attribution error and its role in prejudice. The ultimate attribution error occurs when ingroup members "(1) attribute negative outgroup behavior to dispositional causes (more than they would for identical ingroup behavior), and (2) attribute positive outgroup behavior to one or more of the following causes: (a) a fluke or exceptional case, (b) luck or special advantage, (c) high motivation and effort, and (d) situational factors."
Youeng-Bruehl (1996) argued that prejudice cannot be treated in the singular; one should rather speak of different prejudices as characteristic of different character types. Her theory defines prejudices as being social defences, distinguishing between an obsessional character structure, primarily linked with anti-semitism, hysterical characters, primarily associated with racism, and narcissistic characters, linked with sexism. 
Contemporary theories and empirical findings
The out-group homogeneity effect is the perception that members of an out-group are more similar (homogenous) than members of the in-group. Social psychologists Quattrone and Jones conducted a study demonstrating this with students from the rival schools Princeton University and Rutgers University.  Students at each school were shown videos of other students from each school choosing a type of music to listen to for an auditory perception study. Then the participants were asked to guess what percentage of the videotaped students' classmates would choose the same. Participants predicted a much greater similarity between out-group members (the rival school) than between members of their in-group.
The justification-suppression model of prejudice was created by Christian Crandall and Amy Eshleman.  This model explains that people face a conflict between the desire to express prejudice and the desire to maintain a positive self-concept. This conflict causes people to search for justification for disliking an out-group, and to use that justification to avoid negative feelings (cognitive dissonance) about themselves when they act on their dislike of the out-group.
The realistic conflict theory states that competition between limited resources leads to increased negative prejudices and discrimination. This can be seen even when the resource is insignificant. In the Robber's Cave experiment, negative prejudice and hostility was created between two summer camps after sports competitions for small prizes. The hostility was lessened after the two competing camps were forced to cooperate on tasks to achieve a common goal.
Another contemporary theory is the integrated threat theory (ITT), which was developed by Walter G Stephan.  It draws from and builds upon several other psychological explanations of prejudice and ingroup/outgroup behaviour, such as the realistic conflict theory and symbolic racism.  It also uses the social identity theory perspective as the basis for its validity; that is, it assumes that individuals operate in a group-based context where group memberships form a part of individual identity. ITT posits that outgroup prejudice and discrimination is caused when individuals perceive an outgroup to be threatening in some way. ITT defines four threats:
- Realistic threats
- Symbolic threats
- Intergroup anxiety
- Negative stereotypes
Realistic threats are tangible, such as competition for a natural resource or a threat to income. Symbolic threats arise from a perceived difference in cultural values between groups or a perceived imbalance of power (for example, an ingroup perceiving an outgroup's religion as incompatible with theirs). Intergroup anxiety is a feeling of uneasiness experienced in the presence of an outgroup or outgroup member, which constitutes a threat because interactions with other groups cause negative feelings (e.g., a threat to comfortable interactions). Negative stereotypes are similarly threats, in that individuals anticipate negative behaviour from outgroup members in line with the perceived stereotype (for example, that the outgroup is violent). Often these stereotypes are associated with emotions such as fear and anger. ITT differs from other threat theories by including intergroup anxiety and negative stereotypes as threat types.
Additionally, social dominance theory states that society can be viewed as group-based hierarchies. In competition for scarce resources such as housing or employment, dominant groups create prejudiced "legitimizing myths" to provide moral and intellectual justification for their dominant position over other groups and validate their claim over the limited resources.  Legitimizing myths, such as discriminatory hiring practices or biased merit norms, work to maintain these prejudiced hierarchies.
Prejudice can be a central contributing factor to depression.  This can occur in someone who is a prejudice victim, being the target of someone else's prejudice, or when people have prejudice against themselves that causes their own depression.
Controversies and prominent topics
One can be prejudiced against or have a preconceived notion about someone due to any characteristic they find to be unusual or undesirable. A few commonplace examples of prejudice are those based on someone's race, gender, nationality, social status, sexual orientation, or religious affiliation, and controversies may arise from any given topic.
Sexism, also called gender discrimination, is prejudice or discrimination based on a person's sex or gender. Sexism can affect either gender, but it is particularly documented as affecting women and girls. The discussion of such sentiments, and actual gender differences and stereotypes continue to be controversial topics. Throughout history, women have been thought of as being subordinate to men, often being ignored in areas like the academia or belittled altogether. Traditionally, men were thought of as being more capable than women, mentally and physically. In the field of social psychology, prejudice studies like the "Who Likes Competent Women" study led the way for gender-based research on prejudice. This resulted in two broad themes or focuses in the field: the first being a focus on attitudes toward gender equality, and the second focusing on people's beliefs about men and women. Today, studies based on sexism continue in the field of psychology as researchers try to understand how people's thoughts, feelings, and behaviors influence and are influenced by others.
Nationalism is a sentiment based on common cultural characteristics that binds a population and often produces a policy of national independence or separatism.  It suggests a "shared identity" amongst a nation's people that minimizes differences within the group and emphasizes perceived boundaries between the group and non-members. This leads to the assumption that members of the nation have more in common than they actually do, that they are "culturally unified", even if injustices within the nation based on differences like status and race exist. During times of conflict between one nation and another, nationalism is controversial since it may function as a buffer for criticism when it comes to the nation's own problems since it makes the nation's own hierarchies and internal conflicts appear to be natural. It may also serve a way of rallying the people of the nation in support of a particular political goal. Nationalism usually involves a push for conformity, obedience, and solidarity amongst the nation's people and can result not only in feelings of public responsibility but also in a narrow sense of community due to the exclusion of those who are considered outsiders. Since the identity of nationalists is linked to their allegiance to the state, the presence of strangers who do not share this allegiance may result in hostility.
Classism is defined by dictionary.com as "a biased or discriminatory attitude on distinctions made between social or economic classes."  The idea of separating people based on class is controversial in itself. Some argue that economic inequality is an unavoidable aspect of society, so there will always be a ruling class.  Some also argue that, even within the most egalitarian societies in history, some form of ranking based on social status takes place. Therefore, one may believe the existence of social classes is a natural feature of society.
Others argue the contrary. According to anthropological evidence, for the majority of the time the human species has been in existence, humans have lived in a manner in which the land and resources were not privately owned. Also, when social ranking did occur, it was not antagonistic or hostile like the current class system. This evidence has been used to support the idea that the existence of a social class system is unnecessary. Overall, society has neither come to a consensus over the necessity of the class system, nor been able to deal with the hostility and prejudice that occurs because of the class system.
One's sexual orientation is the "direction of one's sexual interest toward members of the same, opposite, or both sexes".  Like most minority groups, homosexuals and bisexuals are not immune to prejudice or stereotypes from the majority group. They may experience hatred from others because of their sexual preferences; a term for such intense hatred based upon one's sexual orientation is homophobia.
Due to what social psychologists call the vividness effect, a tendency to notice only certain distinctive characteristics, the majority population tends to draw conclusions like gays flaunt their sexuality. Such images may be easily recalled to mind due to their vividness, making it harder to appraise the entire situation. The majority population may not only think that homosexuals flaunt their sexuality or are "too gay", but may also erroneously believe that homosexuals are easy to identify and label as being gay or lesbian when compared to others who are not homosexual. 
The idea of heterosexual privilege seems to flourish in society. Research and questionnaires are formulated to fit the majority; i.e., heterosexuals. This discussion of whether heterosexuals are the privileged group and whether homosexuals are a minimized group is controversial. Research shows that discrimination on the basis of sexual orientation is a powerful feature of many labor markets. For example, controlling for human capital, studies show that gay men earn 10% - 32% less than heterosexual men in the United States, and that there is significant discrimination in hiring on the basis of sexual orientation in many labor markets. 
Racism is defined as the belief that physical characteristics determine cultural traits, and that racial characteristics make some groups superior. By separating people into hierarchies based upon their race, it has been argued that unequal treatment among the different groups of people is just and fair due to their genetic differences. Racism can occur amongst any group that can be identified based upon physical features or even characteristics of their culture. Though people may be lumped together and called a specific race, everyone does not fit neatly into such categories, making it hard to define and describe a race accurately.
Scientific racism began to flourish in the eighteenth century and was greatly influenced by Charles Darwin's evolutionary studies, as well as ideas taken from the writings of philosophers like Aristotle; for example, Aristotle believed in the concept of " natural slaves ". This concept focuses on the necessity of hierarchies and how some people are bound to be on the bottom of the pyramid. Though racism has been a prominent topic in history, there is still debate over whether race actually exists, making the discussion of race a controversial topic. Even though the concept of race is still being debated, the effects of racism are apparent. Racism and other forms of prejudice can affect a person's behavior, thoughts, and feelings, and social psychologists strive to study these effects.
While various religions teach their members to be tolerant of those who are different and to have compassion, throughout history there have been wars, pogroms and other forms of violence motivated by hatred of religious groups.
In the modern world, researchers in western, educated, industrialized, rich and democratic countries have done various studies exploring the relationship between religion and prejudice; thus far, they have received mixed results. A study done with US college students found that those who reported religion to be very influential in their lives seem to have a higher rate of prejudice than those who reported not being religious. Other studies found that religion has a positive effect on people as far as prejudice is concerned. This difference in results may be attributed to the differences in religious practices or religious interpretations amongst the individuals. Those who practice "institutionalized religion", which focuses more on social and political aspects of religious events, are more likely to have an increase in prejudice. Those who practice "interiorized religion", in which believers devote themselves to their beliefs, are most likely to have a decrease in prejudice.
Individuals or groups may be treated unfairly based solely on their use of language. This use of language may include the individual's native language or other characteristics of the person's speech, such as an accent, the size of vocabulary (whether the person uses complex and varied words), and syntax. It may also involve a person's ability or inability to use one language instead of another. In the mid-1980s, linguist Tove Skutnabb-Kangas captured this idea of discrimination based on language as the concept of linguicism. Kangas defined linguicism as the ideologies and structures used to "legitimate, effectuate, and reproduce unequal division of power and resources (both material and non-material) between groups which are defined on the basis of language."
Broadly speaking, attribution of low social status to those who do not conform to neurotypical expectations of personality and behaviour. This can manifest through assumption of 'disability' status to those who are high functioning enough to exist outside of diagnostic criteria, yet do not desire to (or are unable to) conform their behaviour to conventional patterns. This is a controversial and somewhat contemporary concept; with various disciplinary approaches promoting conflicting messages what normality constitutes, the degree of acceptable individual difference within that category, and the precise criteria for what constitutes medical disorder. This has been most prominent in the case of high-functioning Autism, where direct cognitive benefits increasingly appear to come at the expense of social intelligence. 
Discrimination may also extend to other high functioning individuals carrying pathological phenotypes, such as those with Attention deficit hyperactivity disorder and Bipolar spectrum disorders. In these cases, there are indications that perceived (or actual) socially disadvantageous cognitive traits are directly correlated with advantageous cognitive traits in other domains, notably creativity and divergent thinking,  and yet these strengths might become systematically overlooked. The case for "neurological discrimination" as such lies in the expectation that one's professional capacity may be judged by the quality of ones social interaction, which can in such cases be an inaccurate and discriminatory metric for employment suitability.
Since there are moves by some experts to have these higher-functioning extremes reclassified as extensions of human personality,  any legitimisation of discrimination against these groups would fit the very definition of prejudice, as medical validation for such discrimination becomes redundant. Recent advancements in behavioural genetics and neuroscience have made this a very relevant issue of discussion, with existing frameworks requiring significant overhaul to accommodate the strength of findings over the last decade.
Humans have an evolved propensity to think categorically about social groups, manifested in cognitive processes with broad implications for public and political endorsement of multicultural policy, according to psychologists Richard J. Crisp and Rose Meleady.  They postulated a cognitive-evolutionary account of human adaptation to social diversity that explains general resistance to multiculturalism, and offer a reorienting call for scholars and policy-makers who seek intervention-based solutions to the problem of prejudice.
The contact hypothesis
The contact hypothesis predicts that prejudice can only be reduced when in-group and out-group members are brought together. In particular, there are six conditions that must be met to reduce prejudice, as were cultivated in Elliot Aronson's "jigsaw" teaching technique. First, the in- and out-groups must have a degree of mutual interdependence. Second, both groups need to share a common goal. Third, the two groups must have equal status. Fourth, there must be frequent opportunities for informal and interpersonal contact between groups. Fifth, there should be multiple contacts between the in- and the out-groups. Finally, social norms of equality must exist and be present to foster prejudice reduction.
Academics Thomas Pettigrew and Linda Tropp conducted a meta-analysis of 515 studies involving a quarter of a million participants in 38 nations to examine how intergroup contact reduces prejudice. They found that three mediators are of particular importance: Intergroup contact reduces prejudice by (1) enhancing knowledge about the outgroup, (2) reducing anxiety about intergroup contact, and (3) increasing empathy and perspective-taking. While all three of these mediators had mediational effects, the mediational value of increased knowledge was less strong than anxiety reduction and empathy.  In addition, some individuals confront discrimination when they see it happen, with research finding that individuals are more likely to confront when they perceive benefits to themselves, and are less likely to confront when concerned about others' reactions.