Sigmund Freud developed psychoanalytic theory and viewed human behavior as being determined by unconscious motivations and childhood experiences. His theory of psychosexual development proposed that personality develops through oral, anal, phallic, latency, and genital stages. Fixation at an early stage can result from over- or under-gratification of needs and influence adult personality and behavior. Freud believed unconscious drives and childhood experiences, like the Oedipus complex, strongly influence development and must be resolved to achieve healthy adulthood.