Philipp Jolly and his 1913 “the heredity of psychosis”: Homogeneity versus heterogeneity of familial transmission and an early look at Mendelian models for manic‐depressive illness and dementia praecox

Philipp Jolly's 1913 extensive monograph “The Heredity of Psychoses” provides, in both his detailed literature review and new pedigree study, an extensive assessment of a key issue in the psychiatric genetics of his day: the degree to which the familial transmission of psychiatric disorders was specific (or homogeneous) versus nonspecific (or heterogeneous). Contrary to a number of earlier observations, Jolly concludes that heterogeneous transmission is rare. Multiple psychiatric disorders can occur in one family because members of a family may have elevated predispositions to more than one disorder rather than one predisposition which increases risk for a wide range of conditions. A notable exception to this, he notes, is within manic‐depressive illness (MDI), where different forms of illness run together within families. Jolly is also among the earliest investigators to evaluate simple Mendelian models for MDI and dementia praecox (DP). While his methods were primitive and contained only “eye‐ball” examinations rather than statistical modeling, Jolly demonstrated considerable conceptual sophistication in the early application of such models. He concludes that DP is likely a recessive disorder. MDI, he states, is either a recessive condition or demonstrates “gender‐based dominant heredity” where heterozygote females are affected, and heterozygote males are not.