Collins Dictionary of Sociology defines identity as the set of qualities, beliefs, personality traits, appearance, or expressions that characterize a person or group. This definition emerged from centuries of philosophical inquiry into what makes a human being distinct.
Erik Erikson lived from 1902 until 1994 and became one of the earliest psychologists to take explicit interest in identity. His theory of psychosocial development introduced the concept of ego identity as an individual's personal sense of continuity.
James Marcia developed his Neo-Eriksonian identity status paradigm in 1966 which includes four possible results: identity diffusion, foreclosure, moratorium, and achievement. Diffusion occurs when a person avoids both exploration and making commitments while achievement requires having explored many possibilities before committing to an identity.
Social factors shape this development alongside cultural influences because how others acknowledge one's characteristics plays a critical role in forming a stable sense of self. Social identity theory shows that crafting cognitive distinction between in-groups and out-groups leads to subtle effects on evaluations of others.
An exclusive boundary arises when a person adopts a marker imposing restrictions on others' behavior whereas inclusive boundaries form through use of markers with which others are ready and able to associate. Markers include language, dress, behavior, and choice of space whose effect varies based on recognition by other social beings.