This paper proposes a critical reflection on the history, paradigm and scientific nature of penile plethysmography, a technical device used to assess sexual delinquency. Its initial objective was medical but it is now devoted to identifying “deviant” sexual tendencies of defendants. Despite the weaknesses of its psychometric properties the value of this technical device indicates a specific conception of sex offenders, seen as lying and hiding their sexual deviance. Its application is based on the theoretical premise that penile tumescence is correlated positively with, and is a consequence of, sexual arousal. The paper is a review of the criticisms we can address in relation to the device and its many problematic postulates in a historic perspective.