Psychiatric genetics is a relatively new term for an old research question: “Are behavioral and psychological conditions and deviations inherited?” The systematic empirical inquiries in this field started in the late nineteenth century with the work of F. Galton and his monograph Talent and Character, which was motivated by Darwin’s theory and the concept of degeneration. During the twentieth century, the methodological standard of the field was improved by the development of epidemiological, biometrical, and clinical research tools. This was the precondition to perform valid family, twin, and adoption studies. These methods revealed that all psychiatric disorders aggregate in families, and that genes influence the manifestation of these disorders. It became clear that the degree of familiality and extent of genetic influence varies among diseases, with schizophrenia showing the strongest genetic background and disorders such as obsessive-compulsive and borderline personality disorder showing the weakest genetic background. Although there is some overlap, the familial patterns of diagnoses reveal a surprisingly high specificity, which was considered an argument for the appropriateness of diagnostic definitions. Considering the limitations in the pathophysiological understanding of psychiatric disorders, “breeding true” of diagnosis in families became the hallmark indicator of clinical validity (1).