To these general conditions must be added the plain and everyday contrast between the metaphysical perfection of criminal law and the progressive increase of crime, as well as the contrast between legal theories of crime and the study of the mental characteristics of a large number of criminals.

Ferri disputed the emphasis on physiological characteristics of criminals; instead, he focused on the study of psychological characteristics, which he believed accounted for the development of crime in an individual. These characteristics included slang, handwriting, secret symbols, literature, and art, as well as moral insensibility.

Enrico Ferri war ein italienischer Kriminologe und Politiker. Er bildete neben Cesare Lombroso und Raffaele Garofalo die sogenannte "scuola positiva" ("positive Schule") der italienischen Kriminologie und wurde somit zu einem der Begründer der modernen Kriminologie.