The bitterness and protracted character of the biometrician–Mendelian debate has long aroused the interest of historians of biology. In this paper, we focus on another and much less discussed facet of the controversy: competing interpretations of the inheritance of mental defect. Today, the views of the early Mendelians, such as Charles B. Davenport and Henry H. Goddard, are universally seen to be mistaken. Some historians assume that the Mendelians' errors were exposed by advances in the science of genetics. Others believe that their mistakes could have been identified by contemporaries. Neither interpretation takes account of the fact that the lapses for which the Mendelian eugenicists are now notorious were, in fact, mostly identified at the time by the biometricians David Heron and Karl Pearson. In this paper we ask why their objections had so little impact. We think the answer illustrates an important general point about the social prerequisites for effective scientific critique.