Today, their research is debunked because of poor methodology, testing, and logic. Biological views fell out of favor in the early 20th century. In the 1970's, spurred by a publication by Wilson on Sociobiology, several criminals again turned to study the biological basis of criminality. For the most part, the effort has focused on the cause of violent crime. Interest has centered on several areas: (1) biochemical factors, such as diet, allergies, hormonal imbalances, and environmental contaminants such as lead; (2) neurophysiological factors, such as brain disorders, EEG abnormalities, tumors, and head injuries; and (3) genetic factors, such as the XYY syndrome and inherited traits. An evolutionary branch holds changes in the human condition that have taken millions of years to evolve may help explain crime rate differences. Psychological attempts to explain criminal behavior have historical roots in Maudsley's concept that all criminals are insane or mentally damaged, but this position is no longer accepted. There are currently three main psychological perspectives--the psycho-dynamic view, the cognitive view, and the social learning view. The psycho-dynamic view links aggressive behavior to personality conflicts developed in childhood. The cognitive view is concerned with human development and how people perceive the world. Behavioral and social learning theories see criminality as a learned behavior. Psychological traits such as personality and intelligence have been linked to criminality. One important area of study has been the psychopath, a person who lacks emotion and concern for others. Psychologists have developed standardized tests to measure personality traits, and one avenue of research has been to determine whether criminals and non-criminals manifest any differences in their responses to test items. 216 notes, 4 tables, 2 figures, and 8 photographs