Psychiatric genetics emerged from a small, but diverse, network that took shape in Germany at the beginning of the twentieth century. Experimental biology, Mendelian statistics and psychiatric classification were styles of reasoning that crystallized in the milieu of the ‘research institute’. Together, they produced a method of calculating Mendelian inheritance of schizophrenia that crossed the threshold of a mature science of heredity. The chapter examines how a new style of testing the truth and falsity of Mendelian inheritance emerges from a small, but diverse, network of biological psychiatry. It considers the ways in which schizophrenia serves as a model for stabilizing the field of psychiatric genetics. The chapter explores how clinical and aetiological heterogeneity are theoretically integrated to create new pathways for research. Psychiatric genetics was a small network of investigators with an empirical vision of estimating the inheritance of severe psychoses. A new science of heredity took shape around developments in nosology, scientific genealogy and methodological diversity.