Sigmund Freud was an Austrian neurologist who founded the psychoanalytic school of psychology. Some of Freud's key contributions included his theories of the conscious, unconscious and preconscious mind; the id, ego and superego model of personality; psychosexual development and the Oedipus complex; defense mechanisms; and the life and death instincts. Freud believed that unconscious drives and early childhood experiences highly influence behavior and psychopathology. He pioneered the use of techniques including free association, dream interpretation, and psychoanalysis. Though some of Freud's theories are no longer considered scientifically valid, he had a massive influence on 20th century psychology and culture.