For millennia, Western legal tradition relied on the assumption “pater semper incertus est” (“the father is always uncertain”). But beginning in the early twentieth century, scientists began a concerted quest for a biological marker of paternity that could unambiguously link a child to his or her progenitor. Prior to the advent of DNA testing, scientists from around the world experimented with fingerprints, dental evidence, ear shape, and blood typing. This study by Nara Milanich, Associate Professor of History at Barnard College, examines the link between the history of the paternity test and the history of gender, family, and changing ideas about kinship. Although today paternity can be determined with over 99% accuracy, biological certainty by no means displaces legal and social constructions of family, and in the age of modern biomedicine, the definition of parentage is arguably as “uncertain” as ever.