Labelling theory (also called “societal reaction theory”) is a social theory that holds that society's reaction to certain behaviours is a major factor in defining the self as deviant. That is, people may become “deviant” because certain labels (thief, prostitute, homosexual) are attached to their behaviour by criminal justice authorities and others. The resulting treatment of the individual pushes them into performing the deviant role.
In social psychology, this is a way of explaining deviant behaviour by focusing on the reactions of other people to the person they label deviant. When combined with self-fulfilling prophecy labelling theory has also been used to account for psychotic behaviour. Thus if a person is labeled paranoid everything she does is interpreted in the light of her paranoia and invariably taken as evidence for the correctness of the original diagnosis.