Kalmoe is a professor of political communications at LSU, and Mason is a professor of political science at Johns Hopkins. The authors approach their topic with rigorous social science, relying heavily on numerous complex surveys collected since 2017. Fittingly, they open with the Jan. 6, 2021, insurrection and return to those deplorable events throughout, and Donald Trump’s name appears countless times in the text, never accompanied by praise. While the authors do not conceal their low opinion of the ex-president and his inflammatory rhetoric and disinformation campaigns, their studies turn up unsettling reasons for his persistent popularity. Kalmoe and Mason emphasize that partisan political strife, including racially motivated violence, has occurred regularly throughout American history, and both government leaders and ordinary citizens have been involved. While social norms forbid seriously harming others, the authors emphasize the process of “moral disengagement” that “cushions the ego” when people contemplate violence. “Moral disengagement,” they write, “includes vilifying out-groups, hyping the morality of in-groups, minimizing harms done, shifting blame, and focusing on righteous ends that justify aggressive means. Among these, vilification is the most potent for producing aggressive behavior.” One of the authors’ most surprising findings may be that “ordinary Democrats and Republicans are remarkably similar in partisan moral disengagement and violent views, despite greater prevalence of right-wing violence.” Still, the events of Jan. 6 vaulted Republicans to a lead in both. The book’s many surveys and charts reveal that individuals who strongly identify with a party are more likely to approve of violence against the other, including behavior associated with “hostile sexism,” which “is only related to moral disengagement among Republicans—and powerfully so—shifting them 40 points up the disengagement scale.” Age has little influence, though college graduates are less likely to approve of violence. This is academic political science, packed with statistics, scholarly language, tables, charts, and footnotes, but it will reward policy wonks who pay close attention.