GONE ARE the days when conspiracy-mongers had to find shards of evidence and contort it to convince people. Now, just their malevolence is needed. If a concocted scenario can’t be proved, then perhaps it can’t be disproved either. That is toxic for a stable society and politics. So how did we get here, and how do we get out?

Nancy L. Rosenblum of Harvard University and Russell Muirhead of Dartmouth College are the authors of “A Lot of People Are Saying: The New Conspiracism and the Assault on Democracy” (Princeton, 2019). Though conspiracy theories have always existed, they note that today something is different and dangerous: “Conspiracy without the theory.”