On the morning of December 4, 2016, Edgar Maddison Welch, a warehouse worker and a father of two from Salisbury, North Carolina, told his family that he had a few things to do; loaded an AR-15, a .38-calibre revolver, and a folding knife into his car; and headed for Washington, D.C. Welch’s intention, he later told police, was to “self-investigate” a plot featuring—in no particular order—Hillary Clinton, sex trafficking, satanic rituals, and pizza.

At around 3 P.M., Welch arrived at Comet Ping Pong, a restaurant in Chevy Chase, where, he believed, children were being held in a network of tunnels. He made his way to the kitchen, shot open a locked door, and discovered cooking utensils. In an interview from jail, a few days later, he acknowledged to the Times, “The intel on this wasn’t a hundred percent.” He’d found no captive children in the restaurant’s basement; in fact, as many accounts of the incident noted, Comet Ping Pong doesn’t even have a basement.

Far from being dissuaded by the new “intel,” believers in what had become known as Pizzagate dug in. Welch had dabbled in acting—he’d appeared as a victim in a low-budget slasher movie—thus, it followed, his raid on the restaurant had been staged. That the plotters had gone to such lengths to cover their tracks showed just how much evil there was to hide. “This shit runs very deep,” a contributor to the subreddit thread r/Conspiracy wrote. All the while, the restaurant’s owner was receiving death threats.

Some ten months after the incident at Comet Ping Pong, a prediction surfaced on the Web that Clinton would soon be arrested. “Expect massive riots organized in defiance,” an anonymous poster, Q, warned on the message board 4chan. Other prophecies followed: Clinton’s campaign chairman, John Podesta, would also be arrested; members of the media would be “jailed as deep cover agents”; there would be a Twitter blackout heralding a government purge.

As Q’s prophecies failed, more converts were won over. QAnon, as Q’s world view came to be known, subsumed—or, if you prefer, consumed—Pizzagate, and then it, too, slunk off the Web and into the world. Last June, an unemployed former marine named Matthew Wright parked a home-built armored truck on the Mike O’Callaghan–Pat Tillman Memorial Bridge, which spans the Colorado River on the border of Arizona and Nevada. Wright, who, like Welch, was armed with an AR-15 and a handgun, blocked traffic for almost ninety minutes before surrendering to police. At one point, he held up a sign that said “Release the OIG report,” a reference to another QAnon prediction, involving the Justice Department’s Office of the Inspector General. Following his arrest, Wright wrote a letter to the White House saying that he “simply wanted the truth on behalf of all Americans, all of humanity for that matter.”

America has always had a weakness for paranoid fantasies. According to some historians, the Founding Fathers were moved to write the Declaration of Independence by groundless fears of a British plot. “Conspiracy Theories in American History,” a two-volume encyclopedia, runs from “Abolitionism” to “ZOG.” (ZOG, an acronym used by survivalists, is shorthand for the “Zionist Occupied Government,” which, the encyclopedia explains, refers to an “international Jewish conspiracy to undermine U.S. sovereignty and true Christianity.”) In between are some three hundred entries, including “Black Helicopters,” “Contrails,” “Illuminati,” “Moon Landings,” “Pan Am 103,” and “Roswell.”

In this context, Pizzagate and QAnon could be considered madness as usual—just two late-alphabet entries in the annals of national crankdom. But is that all there is to it? Or are deeper, darker forces at work? A confirmed conspiracist now occupies the White House and, “no collusion” notwithstanding, there’s evidence that an international conspiracy put him there. Coincidence? To paraphrase Q, perhaps it’s time to “expand our thinking.”

Russell Muirhead and Nancy L. Rosenblum are professors of government at, respectively, Dartmouth and Harvard. A few years ago, they found themselves, in their words, “startled into thought.” Yes, they knew, crazy ideas were a fixture of American life. But not this crazy. “The subject required more detailed and thoughtful interpretation,” the two write at the beginning of “A Lot of People Are Saying: The New Conspiracism and the Assault on Democracy.”

“Classic” conspiracy theories, according to Muirhead and Rosenblum, arise in response to real events—the assassination of John F. Kennedy, say, or the terrorist attacks of September 11th. Such theories, they argue, constitute a form of explanation, however inaccurate they may be. What sets theories like QAnon apart is a lack of interest in explanation. Indeed, as with the nonexistent child-trafficking ring being run out of the nonexistent basement, “there is often nothing to explain.” The professors observe, “The new conspiracism sometimes seems to arise out of thin air.”

The constituency, too, has shifted. Historically, Muirhead and Rosenblum maintain, it’s been out-of-power groups that have been drawn to tales of secret plots. Today, it’s those in power who insist the game is rigged, and no one more insistently than the so-called leader of the free world.

Donald Trump got his start in national politics as a “birther,” promoting the idea that President Barack Obama wasn’t born in the United States. Several news organizations have tried to keep track of the conspiracy theories Trump has floated since then. One list, posted by the Web site Business Insider, has nineteen entries. These include the claims that vaccines can cause autism and that Supreme Court Justice Antonin Scalia may have been murdered.

“They’re saying they found the pillow on his face,” Trump said of Scalia, during the 2016 campaign, “which is a pretty unusual place to find a pillow.” (The Business Insider list is limited to full-blown conspiracy theories, and excludes the President’s more casual lies and fabrications.) “No president—indeed, no national official—has resorted to accusations of conspiracy so instinctively, so frequently, and with such brio as Donald Trump,” Muirhead and Rosenblum write.

With Trump in power, they worry, there’s a danger that his dark fantasies may be realized. Democracies depend on buy-in; citizens need to believe in certain basics, starting with the legitimacy of elections. Trump both runs the government and runs it down. The electoral system, he asserts, can’t be trusted. Voter fraud is rampant. His contempt for institutions ranging from the courts (“slow and political”) to the Federal Communications Commission (“so sad and unfair”) to the F.B.I. (“What are they hiding?”) weakens those institutions, thereby justifying his contempt. As government agencies “lose competence and capacity, they will come to look more and more illegitimate to more and more people,” Muirhead and Rosenblum observe.

Trump is so closely tied to the “new conspiracism” that it can be hard to tell the ranter from the rant. Then again, it’s hard to imagine his ascent without other key developments: the polarization of the electorate, a generation of attacks (mostly from the right) on the news media and government, and, of course, the rise of the Web. Spreading conspiracy theories once had a price—printing or even mimeographing a tract costs money—but now, as Muirhead and Rosenblum point out, anyone can post a madcap theory or a doctored photograph virtually for free.