Medical genetics in the 19th century as background to the development of psychiatric genetics.

This article examines the relationship between the early efforts of alienists to understand the role of heredity in the etiology of insanity in the 19th century and the parallel efforts of the nascent discipline of medical genetics. I review three monographs on general medical genetics: Adams in 1814, Steinau in 1843, and Lithgow in 1889. Numerous parallels were seen between their writings and those of their contemporary alienists working on mental disorders including (i) an emphasis on the transmission of the liability to illness rather than the illness itself, (ii) discussions of the homogeneous versus heterogeneous nature of familial transmission of disease, (iii) the relative value of direct versus indirect hereditary effects, (iv) the role of mothers versus fathers in transmitting liability, (v) possible environmental sources of familial clustering, and (vi) the transmission of age at onset of illness. All three medical genetic authors noted that insanity was among the more heritable of human disorders. Furthermore, Lithgow noted the importance of heritable influences on the non-psychotic forms of psychiatric illness rarely seen in asylums. This survey demonstrates substantial consilience in the topics of interest and conclusions of the nascent general medical and psychiatric genetics' communities in the 19th century.