Throughout the 19th century, many alienists reported the proportion of their patients who were “hereditarily burdened,” meaning they had a positive family history for mental illness. The rates of such burden differed widely because different authors used divergent definition of illness and investigated different groups of relatives. Most importantly, no authors compared rates of burden with those seen in a nonpatient control group. The first such study in the history of psychiatric genetics was published in 1895, the doctoral dissertation of a Swiss physician Jenny Koller working under Auguste Forel. She obtained histories of a range of mental/neurologic disorders in the parents, aunts/uncles, grandparents and siblings of 370 hospitalized psychiatric patients and 370 controls. Rates of any hereditary burden were only modestly higher in cases (78%) than controls (59%). However, when examining individual syndromes, only major mental illness and eccentricities, but not apoplexy, nervous disorders or dementia, were more common in proband than control families. Furthermore, the rates of mental illness and eccentricities were substantially elevated in the first‐degree relatives of cases versus controls but not in the second‐degree relatives. Koller's study represented a major methodological advance in psychiatric genetics, helping to define which disorders coaggregated with major mental illness.

Authored by one of Rüdin’s most productive colleagues, Bruno Schulz, the main goal of this article was to estimate risk for dementia praecox (DP) in nieces/nephews (n/n) of DP probands to address practical questions for genetic counseling. Schulz selected 76 primary probands from records of the genealogical department of the German Research Institute of Psychiatry who both had DP and had siblings—secondary probands—with children aged 20 years or greater. Those children had a morbid risk for DP of 1.4%, much lower than that found in the offspring of DP in Hoffmann’s study. However, the secondary probands had much lower rates of DP than the unselected siblings of DP cases studied by Rüdin. So rates of DP in n/n of DP probands are likely lower than expected theoretically because the secondary probands are being indirectly selected for a lowered genetic risk for DP by having successfully reproduced. Schulz diagnosed eccentric and other disordered personalities in the secondary probands and their spouses and presented evidence that other disordered personalities but especially eccentric personalities in parents were related to the risk of both eccentricity and DP in their children, the n/n of the DP probands. Eugenic issues play a minor role in this article. Although generally supportive of eugenic goals, historical sources document that Schulz was opposed to the National Socialists and actively attempted to protect Jews and his patients from involuntary sterilization. This complicates attempts to judge the morality of the work of the Rüdin school as a single unit.

In 1916, Ernst Rüdin published the first modern family study in the history of psychiatric genetics, the major goal of which was to test whether the pattern of risk in the siblings of dementia praecox (DP) probands followed Mendelian expectations. He utilized systematic ascertainment of probands and multisourced diagnostic assessments of probands and relatives, applying the narrow Kraepelinian concept of DP. In a novel step, he collaborated closely with a statistical geneticist—Wilhelm Weinberg—and applied his sibling, proband, and age correction methods. In his key sample—701 sibships when neither parent had DP—the morbid risk for DP in siblings was 4.48%, much lower than 25% expected for a recessive disorder. Risk for DP was increased by alcoholism or other mental disorders in parents. Other non-DP psychoses were common in both siblings and parents of DP probands. Rüdin discussed several alternative genetic models for DP including a 2-locus recessive, incomplete penetrance, and an oligogenic model. The high rates of other psychoses and psychopathic personalities in relatives might arise, he suggested, because these disorders shared genetic risks with DP. Rüdin established that DP, when carefully studied, ran in families, did not have a simple Mendelian genetic transmission pattern, and appeared likely to be genetically related to other non-DP psychotic disorders and perhaps some kinds of psychopathic personalities. This study, the most important in Rüdin’s career, should be viewed in the context of his later extensive support of and collaboration with Nazi eugenic policies.

After decades of methodological stasis in 19th century psychiatric genetics, when uncontrolled studies reported high rates of hereditary burden in hospitalized patients, Koller completed the first controlled study in 1895. We pick up this narrative 7 years later when the well‐known Julius Wagner v. Jauregg published a biting critique of the then current psychiatric genetics' literature. In 1905, partially in response to Wagner v. Jauregg, Otto Diem attempted to replicate and extend Koller's study. Wagner v. Jauregg then wrote a follow‐up to his earlier critique in 1906, commenting on Diem's investigation. Themes discussed in this point‐counterpoint included the necessity of statistical methods to draw meaningful conclusions about the impact of hereditary burden on mental illness, the required sample size and proper selection of controls, the classes of relatives which should optimally be studied, the problems of obtaining accurate information on familial illnesses, the nature of the disorders in families which contribute to mental illness risk and the common unquestioned dogmatic belief that insanity is very often due to hereditary causes. Both Wagner v. Jauregg and Diem spoke out forcefully against the common assumption that hereditary burden operated in a deterministic fashion and emphasized the need to consider other causes of illness.