Social Identity Theory is one of the most influential theories in social psychology for understanding how individuals perceive themselves and interact with others in society. It was developed by Henri Tajfel and John Turner in the 1970s to explain the psychological basis of intergroup relations, prejudice, discrimination, and group behavior. According to this theory, a person's identity is derived not only from personal characteristics but also from the social groups to which they belong. These groups may include family, caste, religion, nationality, occupation, gender, language, ethnicity, or political affiliation. Membership in these groups provides individuals with a sense of belonging, self-esteem, and social identity. The theory suggests that individuals classify themselves and others into different social categories. This process is known as social categorization. Categorization helps people organize the social world by placing individuals into groups based on common characteristics. While categorization simplifies social interaction, it also creates distinctions between "us" (the in-group) and "them" (the out-group).