[10] That year he wrote his most important and influential work, L'uomo delinquente (Criminal Man in English), which went through five editions in Italian and was published in various European languages.

He postulated that criminals represented a reversion to a primitive or subhuman type of person characterized by physical features reminiscent of apes, lower primates, and early humans and to some extent preserved, he said, in modern "savages".

[13][14][15][16] Through years of postmortem examinations and anthropometric studies of criminals, the insane, and normal individuals, Lombroso became convinced that the "born criminal" (reo nato, a term given by Ferri) could be anatomically identified by such items as a sloping forehead, ears of unusual size, asymmetry of the face, prognathism, excessive length of arms, asymmetry of the cranium, and other "physical stigmata".

Although he gave some recognition in his later years to psychological and sociological factors in the etiology of crime, he remained convinced of and identified with, criminal anthropometry.

After he died, his skull and brain were measured according to his own theories by a colleague as he requested in his will; his head was preserved in a jar and is still displayed with his collection at the Museum of Psychiatry and Criminology in Turin.

[21] His notions of physical differentiation between criminals and non-criminals were seriously challenged by Charles Goring (The English Convict, 1913), who made elaborate comparisons and found insignificant statistical differences.

[24] Lombroso's psychiatric theories were conglomerated and collectively called the positivist school by his followers,[24] which included Antonio Marro and Alfredo Niceforo.

Ideas similar to Lombroso's assessment of white and northern-European supremacy over other races would be used by fascists to gird, for example, the promulgation of Italian racial laws.

[24] Through his various publications, Lombroso established a school of psychiatry based on biological determinism and the idea that mental illness was via genetic factors.

Lombroso's theory has been cited as possibly "the most influential doctrine" in all areas studying human behaviour, and indeed, its impact extended far and wide.

[22] According to Lombroso, criminal appearance was not just based on inherited physiognomy such as nose or skull shape, but also could be judged through superficial features like tattoos on the body.

[26] Many adherents to Lombroso's positivist school stayed powerful during Mussolini's rule, because of the seamless way criminal atavism and biological determinism justified both the racial theories and eugenic tendencies of fascism.

[24] His work sponsored the creation of institutions where the criminally insane would be treated for mental illness, rather than placed in jails with their saner counterparts.

[24] In addition to influencing criminal atavism, Lombroso wrote a book called Genio e Follia, in which he discussed the link between genius and insanity.

[23] This hypothesis led to his request to examine Leo Tolstoy for degenerate qualities during his attendance at the 12th International Medical Congress in Moscow in 1897.

[23] Towards the end of his life, Lombroso began to study pellagra, a disease which Joseph Goldberger simultaneously was researching, in rural Italy.

[27] In his attempts to develop these notions, while in Moscow in 1897 he travelled to Yasnaya Polyana to meet Leo Tolstoy in hopes of elucidating and providing evidence for his theory of genius reverting or degenerating into insanity.

[28] Lombroso supplemented these psychological observations with skeletal and cranial measurements, including facial angles, "abnormalities" in bone structure, and volumes of brain fluid.

[29] Lombroso's reference to skull measurements was inspired by the phrenological work and research of German doctor Franz Joseph Gall.

[30] In commenting on skull measurements, Lombroso made observations such as, "I have noted several characters which anthropologists consider to belong to the lower races, such as prominence of the styloid apophysis".

[29] Lombroso connected geniuses to various health disorders as well, by listing signs of degeneration in chapter two of his work, some of which include abnormalities and discrepancies in height and pallor.

[29] Other physical afflictions that Lombroso associated with degeneracy included rickets, emaciation, sterility, lefthandedness, unconsciousness, stupidity, somnambulism, smallness or disproportionality of the body, and amnesia.

[29] In his explanation of the connection between genius and the "degenerative marker" of height, Lombroso cites the following people: Robert and Elizabeth Browning, Henrik Ibsen, George Eliot, Thiers, Louis Blanc and Algernon Charles Swinburne, among others.

In a review of The Man of Genius they stated, "Here we have an hypothesis claiming to be the result of strict scientific investigation and reluctant conviction, bolstered by half-told truths, misrepresentations and assumptions.

[37] Historian Daniel Pick argues that Lombroso serves "as a curious footnote to late-nineteenth-century literary studies", due to his referencing in famous books of the time.