The search for biomarkers of psychiatric illness has a long and checkered history. Most studies have focused on neuroanatomical, biochemical, or functional brain measures. Despite real progress in recent years, overall results have been modest.1 Similarly, individual genetic markers show small effects that are not consistent in every sample.2 These realities have helped promote a move away from existing diagnostic categories toward dimensional measures.3 Thus, it is all the more surprising that a particular kind of genetic assay, known as the polygenic risk score (PRS), has proven to be 1 of the most robust and informative biomarkers of categorical psychiatric diagnoses seen to date.4