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 due to the ideology of the Nazis. 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, in his classic work The Nature of Prejudice, 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.