Sigmund Freud, the founder of psychoanalysis, proposed a theory of personality development through five psychosexual stages: oral, anal, phallic, latency, and genital. Freud believed that sexual energy (libido) is expressed differently at each stage, focusing on specific erogenous zones. Frustration or pleasure in these zones shapes personality, with life being built around the tension and release of libido.

In the oral stage, the mouth is the primary erogenous zone, and pleasure comes from oral stimulation like sucking and tasting. This stage is crucial for developing trust and comfort through interactions with caregivers. The main conflict is weaning, which teaches the child about loss and delayed gratification, leading to independence and trust. Too much or too little gratification can lead to an 'oral stage fixation,' resulting in traits like passivity, gullibility, or immaturity.

The anal stage focuses on the bowels and bladder, with toilet training being the key experience. This stage introduces the conflict between the id's desire for immediate gratification and the ego's demand for delayed gratification. An ideal resolution involves adjusting to moderate parental demands, fostering cleanliness and self-control. Immoderate demands can lead to a compulsive personality, while yielding to the id can result in a self-indulgent personality.

During the phallic stage, the genitalia become the erogenous zone. Children become aware of anatomical sex differences, leading to the Oedipus complex (boys desiring their mother and rivaling their father) and the Electra complex (girls having similar feelings). These complexes are resolved through identification with the same-sex parent, where the child adopts their characteristics.

The latency stage is a period where the libido is dormant, and psychosexual development is repressed. Sexual impulses are sublimated into other activities like schoolwork, hobbies, and same-gender friendships. Children channel their energy into developing new skills and acquiring knowledge, with play becoming increasingly gender-segregated.

The final stage, the genital stage, begins at puberty and extends into adulthood. It involves adolescent sexual experimentation and ideally leads to stable, loving relationships. Sexual instinct is directed towards heterosexual pleasure, transitioning from self-pleasure. Fixations from earlier stages can result in sexual perversions, hindering the proper outlet of sexual instincts. This stage represents most of a person's life, where they confront and resolve childhood psychosexual conflicts, and the ego is established, shifting concern from primary drive gratification to symbolic and intellectual desire.