The ability of several psychometric instruments to differentiate between criminal and non-criminal subjects was investigated. The subjects in the study consisted of fifty male individuals between the ages of 18 and 55, half of which had been convicted of one crime and half of which had no history of criminal activity. The tests administered consisted of the Psychopathic Deviation Scale from the Minnesota Multiphasic Personality Inventory (MMPI), the Psychopathic Deviation Scale of the Clinical Analysis Questionnaire, and two tests designed by the author. The author's tests consisted of the Test of Criminal Cognitions which evaluated antisocial thought patterns and cognitive flexibility, and the Social Semantics Test which assessed individual role definitions. The Test of Criminal Cognitions was administered as a part of a structured interview, and all other scales were administered in a paper and pencil format. The results indicated that the Psychopathic Deviation Scale of the MMPI, and a portion of both the Test of Criminal Cognitions and the Social Semantics Scales differentiated between the groups at the .05 level or better. These findings indicated that criminals tend to be significantly less flexible in their thought and tend to view others in a much more narcissistic manner than non-criminals. The results also indicated that these tests can be utilized to discriminate between criminals and non-criminals. It was additionally noted that the Psychopathic Deviation Scale of the Clinical Analysis Questionnaire did not consistently differentiate between the groups and should not be considered a valid instrument for discriminating between these groups.