In the 1980s, three neurologists from Boston and Glasgow proposed that brain laterality and numerous human ‘abnormalities’ might have a shared hormonal cause. Norman Geschwind (1926–1984) and colleagues suggested that fetal testosterone regulates brain asymmetry and impacts other characteristics including hand preference, sexual orientation, mental abilities, the immune system, and the susceptibility to psychiatric illness. The neurologists produced no experimental data to back their hypothesis. They primarily based their model on a review of hundreds of publications from a wide range of times, places, and disciplines. My paper illustrates that the neurologists mobilized incoherent concepts of ‘the brain’ by drawing on such a variety of scholarly literature. The reviewed works promoted anatomical, hormonal, or genetic understandings of ‘the brain’, and each of these epistemic versions of ‘the brain’ had been assessed with distinct methods, ranging from lesion studies through questionnaires to behavioral observation. Geschwind and colleagues glossed over these substantial conceptual differences in their attempt to distill the heterogeneous literature into a grand unified theory of human life. Drawing from archival collections, published records, and oral history interviews, I show that the epistemic multiplicity of ‘the brain’ led to a tension between concepts of fixity (‘nature’) and plasticity (‘nurture’) in the neurologists’ understanding of the human. I argue that Geschwind and colleagues depended on testosterone to bridge these epistemic divides. Conceiving of this hormone as genetically regulated with anatomically localizable effects, the neurologists combined concepts of fixity and plasticity in a model that essentialized human character and behavior.
