The effort to frame a science of human heredity goes back to about 1810, when it was overwhelmingly based on data on "presumed causes" of insanity and on family pedigrees of asylum patients. It remained highly dependent on institutionalized data of this kind in 1940 (and still). This talk takes up as a transnational problem the Mendelian moment of human/medical genetics, which began about 1908 and faced powerful critiques by 1914. I examine an English medical debate involving William Bateson and Karl Pearson over statistics and Mendelism, then the controversy over Mendelian factors for "psychopathic make-up" and feeble-mindedness identified by friends and allies of the Eugenics Record Office at Cold Spring Harbor. I finish by analyzing the move from Mendelian genetics to "empirical hereditary prognosis" at the German Research Institute for Psychiatry in Munich, and how this played out during the implementation of the Nazi sterilization law.