Freud was a pioneer in the field of psychology. He introduced concepts that had never been considered before, such as the importance of childhood experiences on development, and the idea of hidden, unconscious forces. He believed that the id, ego, and superego are responsible for personality development, as well as undesirable personality issues or traits that may arise in adulthood. Freud's theories also included the idea of defense mechanisms and childhood psychosexual stages. Although originally within his inner circle, prominent Neo-Freudians such as Carl Jung, Alfred Adler, and Karen Horney eventually began to disagree with him on the overemphasis of sex in the psychoanalytic approach.