Italian physician Cesare Lombroso (1918) is the recognized pioneer of the biological school of thought in the study of criminality. In his 1876 book, "Criminal Man," Lombroso first advanced his theory of atavism, which held that criminals are biological degenerates or "throwbacks" to primitive genetic forms. Criminals were identified by Lombroso as having physical traits similar to primitive humans. Although Lombroso later modified some of his hypotheses, they were still rejected by most scientists as biased and unscientific. The debate over the relationship between body type and deviant behavior was revived in the late 1930's by Ernest Hooton (1939). Hooton studied the physical characteristics of thousands of inmates and non-inmates and concluded that the majority of criminals were both physically and mentally inferior to non-criminals. Body-type theories of criminality have been dismissed as failing to show a physiological relationship between body type and delinquent behavior. The role heredity has in delinquent and criminal behavior has long been studied by biological criminologists in attempting to relate criminality to genetics. Early theories such as Dugdale (1877) and Goddard (1914) documented the long histories of deviance in some families, including delinquency, prostitution, idiocy, feeblemindedness, and fornication; however, most modern researchers tend to relate biological factors in criminality and delinquency to multiple causes that include sociologically based factors. Contemporary biological research on delinquency has focused on behavioral patterns of twins, adoption and fosterling studies, the XYY chromosome and criminality, and brain disorders. Current biological studies of juvenile delinquency and criminal behavior are focusing on research efforts in multiple fields, including heredity, biochemistry, immunology, neuroscience, and endocrinology. This multidisciplinary study of biology and deviance is important in providing a better understanding of the overall forces that can increase or decrease susceptibility to biological influences on criminality. The most promising approaches have incorporated biological variables with psychological and sociological variables in explaining juvenile deviant behavior.