Sigmund Freud's theory on schizophrenia was based on the idea that it was caused by unresolved internal conflicts from early childhood, particularly related to the individual's relationship with their mother. He believed that schizophrenia resulted from the inability to manage these conflicts, leading to disintegration of the ego and delusional thinking. Freud's psychoanalytic approach to schizophrenia has been largely criticized and is not widely accepted in the field of psychological research and practice.