Since the diagnosis of “murderous monomania”, of the first decades of the Nineteenth century, to explain the relationship between crime and madness in killings for no apparent reason, and through the issue of instinct, a monstrous and pathological force, which bursts unpredictably in the psyche of the individual, psychiatry changes its way to categorize mental illness and shifts its focus from the disturbance of thought to that of the will and behaviour. If at first the individual notion of social danger as a hereditary defect of a deviant individual, as moral and physical abnormality to isolate and eliminate takes place, then the notion of collective risk, as combinatorial of negative, social and environmental factors prevails, making it possible for particular categories of individuals the occurrence of undesirable behaviours which require the adoption of actuarial, statistical methods, for the purposes of an alleged and predictive evaluation of a safe maintenance of society.

Keywords: Instinct; Abnormality; Dangerousness; Risk; Actuarial Methods.