System justification theory is a theory within social psychology that system-justifying beliefs serve a psychologically palliative function. It proposes that people have several underlying needs, which vary from individual to individual, that can be satisfied by the defense and justification of the status quo, even when the system may be disadvantageous to certain people. People have epistemic, existential, and relational needs that are met by and manifest as ideological support for the prevailing structure of social, economic, and political norms. Need for order and stability, and thus resistance to change or alternatives, for example, can be a motivator for individuals to see the status quo as good, legitimate, and even desirable.

According to system justification theory, people desire not only to hold favorable attitudes about themselves (ego-justification) and the groups to which they belong (group-justification), but also to hold positive attitudes about the overarching social structure in which they are entwined and find themselves obligated to (system-justification). This system-justifying motive sometimes produces the phenomenon known as out-group favoritism, an acceptance of system-consistent oppressive beliefs and stereotypes among low-status groups and viewing higher-status groups in a relatively positive image. Thus, the notion that individuals are simultaneously supporters and victims of system-instilled norms is a central idea in system justification theory. Additionally, the passive ease of supporting the current structure, sustained by the palliative function of system-justifying beliefs that overstate the perceived costs (material, social, or psychological) of altering the status quo, leads to resistance in which the existing social, economic, and political arrangements tend to be preferred. Alternatives to the status quo tend to be disparaged, and inequality continues to perpetuate.

Previous social psychological theories that aimed to explain intergroup behavior typically focused on the tendencies for people to have positive attitudes about themselves (ego-justification) and their self-relevant groups (group-justification). In other words, people are motivated to engage in behaviors that allow for them to maintain a high self-esteem and a positive image of their group. System Justification theory addressed the additional, prevalent phenomenon known as out-group favoritism, in which people defend the social systems (status quo) even when it does not benefit, and in the long-run may even cause more harm, to the individual or the group to which he or she belongs. Out-group favoritism can manifest as a dis-identification on the part of members of lower social status with their own categorical grouping (social, ethnic, economic, political) and instead further support for the existing structure. Prior social psychology theories lacked explanation for and attention given to popular instances of out-group favoritism; thus, system justification theory was developed to further explain and understand why some people tend to legitimize the prevailing social systems, despite their being against one's interests, in a way that previous social psychological theories did not.

While social identity theory, cognitive dissonance theory, the just-world fallacy, social dominance theory, and Marxist-feminist theories of ideologies have heavily influenced system justification theory, it has also expanded on these perspectives, infusing them with the system-justification motive and behaviors.

One of the most popular and well-known social psychological theories, cognitive dissonance theory explains that people have a need to maintain cognitive consistency in order to retain a positive self-image. System justification theory builds off the cognitive dissonance framework, in that it posits people will justify a social system in order to retain a positive image of that social system, due to the fact that they inherently play a role (whether passive or active) in perpetuating it.

Jost and colleagues interpret social identity theory as suggesting that when people are presented with intergroup conflict that threatens their social group identities people will justify behaviors like stereotyping and discrimination against outgroups in order to maintain their positive group image. People with out-group favoritism will hold more positive images of other, often higher-status, groups (outgroups) than the groups they belong to (ingroups). Thus, the argument is that system justification theory builds on the foundations of social identity theory in attempting to account for the out-group favoritism observed in many disadvantaged group members that social identity theory does not.

This theory has widely been compared to system justification theory since they are both system-justifying theories. Social dominance theory focuses on people's motive to maintain a positive group image by generally supporting hierarchical inequality at the group level. Individuals with a high social dominance orientation (SDO) will hold myths that tend to be hierarchy-enhancing, which justify an in-group's place and their relation to it. Thus, in both social dominance theory and system justification theory, there are common threads of group-based opposition to equality and justification for maintaining intergroup inequalities through systemic norms.

According to the just world fallacy, people are inclined to believe the world is generally fair, and that the outcomes of people's behavior are subsequently deserved. Ideologies that relate to belief in a just world have to do with maintaining a sense of personal control and a desire to understand the world as non-random. Such ideologies include the Protestant work ethic and belief in meritocracy. Essentially, belief in a just world feeds an epistemic need for predictability, order, and stability in one's environment. System justification theory, while keeping the perspective that people are inclined to believe the world is just, extracts the underlying epistemic needs of the just world ideology and uses them as support for why people are motivated to uphold the system. In other words, preference for stability, predictability, and the perception of personal control, over random chance, motivates one to see the status quo as fair and legitimate. This can be an issue, however, due to the fact that disadvantaged people can easily internalize their low position and blame themselves for 'shortcomings' or lack of 'successes'.