James Marcia is a prominent psychologist best known for his work on identity development during adolescence. He expanded on Erik Erikson's theories and introduced the concept of identity statuses, which describe how individuals explore and commit to their personal beliefs and values. Marcia identified four identity statuses: identity diffusion, identity foreclosure, identity moratorium, and identity achievement. These statuses help to understand how young people navigate their sense of self and make decisions about their future, contributing significantly to the field of developmental psychology.