" he mentions that boys being spanked may experience incidental penile sensations (if held face down across the lap of the adult, or simply from friction against their own trousers), and consequently come to associate erotic pleasure with spanking, leading to a spanking fetish, and perhaps in some cases to sadism or masochism. "

― Richard von Krafft-Ebing , Psychopathia Sexualis: A Medico-Legal Study