Following the premiere of Tosca in January 1900, Giacomo Puccini's progressive critics generally took issue with two main aspects of the opera: the first was the composer's supposedly unoriginal modes of expression, and the second was the work's scandalous plot. While many attributed the dark tone of Tosca to its French source, Sardou's melodrama La Tosca, I contend that there is an underlying context for both the dramatic and the musical unsavouriness of Puccini's verismo opera: the Italian fascination with criminology. Beginning in the 1870s after Italian unification, positivist criminologists, led by Cesare Lombroso, sought to locate the organic causes of criminality and believed that deviancy was objectively readable through the body. Lombroso further conceptualised the ‘born criminal’ as an exceptional individual that was predisposed to artistic expression. His theories, rooted in deeply troubling stereotypes and conventional wisdom, gained traction with a bourgeois public as well as with contemporary luminaries, including Giuseppe Giacosa, one of Puccini's librettists. Drawing on Lombroso's writings, letters and archived objects, I show how the criminologist's bourgeois version of perversity provides a valuable framework to evaluate the derivative modes of deviant expression present not only in Tosca, but within verismo opera at large.