During the late 1970s and early 1980s, population geneticists sought computational solutions to integrate greater numbers of genetic traits into their debates about the ancestral relationships of human groups. At the same time, geneticists’ longstanding assumptions about Jewish communities, especially Ashkenazim, were challenged by a series of social, political, and intellectual developments. In Israel, the entrenched cultural and political dominance of Ashkenazi Jews faced major social upheaval. Meanwhile, to counteract lingering anti-Semitism in Europe and the United States, Arthur Koestler’s The (...) Thirteenth Tribe and Raphael Patai and Jennifer Patai Wing’s The Myth of the Jewish Race argued that Jewish identity was not connected to biological ancestry from the ancient Israelites. Drawing on scientific publications and archived correspondence, this article reconstructs a transnational social history showing how geneticists responded to these shifting claims about Ashkenazi identity and ancestry. Many argued that these claims could be tested using new statistical models, which provided allegedly more “objective” estimates of ancestral gene frequencies and histories of population admixture. However, they simultaneously engaged in heated debates over the relative superiority of competing statistical approaches. These debates reveal how the transnational reverberations of Israeli ethnic politics and Euro-American anti-Semitism affected the development of new calculations for genetic admixture, permanently shifting the assumptions of population genetic research on Jewish populations as well as other human groups. (shrink)