Con artists are having a moment. The latest crop features Elizabeth Holmes, the deep-voiced, unblinking leader of the fraudulent blood-testing start-up Theranos, one of many cheats now starring in books, podcasts, and dramatic television serials, as if they had accomplished something.

Perhaps they have. Like Gatsby, Holmes invented her celebrity and fortune out of virtually nothing: a smile, a wide and worshipful gaze, and genealogy. A Cincinnati hospital had been named for her great-great-grandfather, a doctor who married into the Fleischmann yeast fortune. One of Holmes’s earliest investors, initially dubious of a young woman who dropped out of college after a year, was impressed by her family history of success in business and medicine, apparently believing such expertise was magically transferrable across generations.

Genealogy, it turns out, has played a rich subterranean part in building improbable expectations, according to Francesca Morgan’s recent book, A Nation of Descendants: Politics and the Practice of Genealogy in US History. Probing the origins of American genealogy, she finds that, from our earliest years, it has been inflating our most grandiose fantasies. While it has served for some peoples—notably African Americans—as a means of recovering their history, finding a sense of belonging, and expanding the country’s social acceptance of once-despised minorities, it has acted at the same time as a tool of exclusion, promoting white supremacy, the Lost Cause, and eugenics. She quotes one “despairing professional” lamenting that “P.T. Barnum missed his calling when he neglected to become a genealogist.”

Yet despite our long fascination with family history, outsiders have also picked up on an American ambivalence toward it. Alexis…