'Crime' and 'criminal' are vague terms relating to behavior deemed undesirable; however, they are subject to interpretation and alteration. The criminal justice system continues to label increasing numbers of people as criminals and to punish them. It is to these persons defined as criminals that researchers have turned to determine what causes crime. Much of the early research in this area was directed toward a biological explanation of criminality. While it has been shown that a few criminals are mentally retarded, have an endocrine imbalance, a chromosomal abnormality, or some other physiological defect, these findings cannot be generalized to a large portion of the criminal population. With advances in psychology throughout the twentieth century, many researchers have focused on personality defects as causes of crime. Yet, like biological theories, psychological theories explain only a small percentage of criminal behavior. Sociological theories of criminal behavior relate crime to environmental and cultural influences that condition behavior. One theory, termed 'labelling', maintains that formal criminal processing so stigmatizes a person that the self-image of being a criminal reinforces criminal associations and behavior. An economic explanation of crime focuses on the rewards certain persons receive from crime compared to a normative lifestyle. None of these theories, however, has yielded treatment methods that ensure reduction in criminal behavior. The amount of crime reported in police statistics is dependent on the proportion of crime reported by victims, which in turn is dependent upon citizen perception of the crimes police focus upon. Five readings are suggested.