The theory that people join social movements because they feel deprived relative to other people or groups with whom they identify.

Relative Deprivation Theory

Explanation:

According to the Relative Deprivation Theory, people who feel like they are deprived of things valued by the society (money, rights, political voice, status)  in comparison to people surrounding them tend to join social movements. These individuals organize or join movements dedicated to obtaining things they feel deprived of. An example of such a movement is the US Civil Rights Movement of the 1960s, organized by Blacks, who wished to gain the same social and legal rights as Whites.