Harry Stack Sullivan was an American psychiatrist who developed an interpersonal theory of personality. He believed that personality is shaped through relationships with others from childhood onward. Sullivan emphasized the importance of early friendships and how they form the basis of personality. His theory viewed personality as being inseparable from the social contexts and interpersonal relationships that people experience. Mental disorders were seen as having interpersonal origins related to problems within social environments. Sullivan made significant contributions to psychotherapy and is considered the first to develop an comprehensive interpersonal theory of personality.