A man holds a Q sign – for the QAnon conspiracy theory – while he waits in line to enter a campaign rally for President Donald Trump in Wilkes-Barre, Pa., on Aug. 2, 2018. AP

Even as we celebrate the 50th anniversary of the lunar landing, truly one of the greatest achievements of the 20th – or any – century, a sizable minority of Americans persist in their belief that the walk on the moon never happened. It was, they say, a put-up job, a conspiracy. Some even embroider the theory to posit that the film director Stanley Kubrick, or someone else, produced the hoax on a sound stage in some back lot in Hollywood.

I confess that I never gave conspiracy theories or theorists much attention, consigning them to the dustbin of flat-earthers or the anti-fluoride brigade. But I began to rethink that during a recent flight from Atlanta to Burlington.

The conversation started out pleasantly enough. My seatmate and I somehow discovered we had Swiss heritage in common. He had recently moved to Utah (“No, I’m not a Mormon”), and he was involved in some computer-type business. He was, in short, an affable, manifestly intelligent and educated person.

The conversation took a turn when he asked what I thought of Donald Trump. I sensed trouble and, not wanting to wade into a political dispute, tried to sidestep the question. He persisted, and I finally mumbled, “Well, not much,” hoping for a quick end to the conversation and an opportunity to bury myself once again in my New Yorker.

No such luck. My interlocutor was unfailingly pleasant, but he proceeded to inform me that Barack Obama was Muslim and that Michelle Obama, his wife, was actually a man. I was dumbfounded. I might have expected such silliness from a boozy lout, with shoes untied and a Phillies jersey, munching on a bag of Cheetos, but this man seemed utterly respectable.

Conspiracy theories have been around for a long time. Thomas Jefferson was accused of being an agent for France when he ran for president in 1800. Ardent Protestants through the centuries have suspected the pope of being the antichrist. Who can forget the Bilderbergers, the Bavarian Illuminati, the Trilateral Commission or the visit of space aliens in Roswell, N.M.? And did you know that the Denver International Airport was built above an underground city, which is the headquarters for the New World Order?

Perhaps it’s my imagination, but it seems to me that conspiracies began to pick up in the 1960s. The John Birch Society had been warning about black helicopters, agents of the United Nations, for a while, but the assassination of John F. Kennedy brought conspiracists into the mainstream – well, almost the mainstream.

Lyndon Johnson was well aware of that possibility, which is why he tapped Earl Warren, chief justice of the U.S. Supreme Court, to head a commission to determine who was responsible for Kennedy’s death. The commission’s finding that Lee Harvey Oswald acted alone did little to placate the fevered conspiracists.

The ’60s prompted a crisis of authority in America. Johnson had lied to us about Vietnam; Richard Nixon lied about, well, pretty much everything. Confidence in institutions – political, educational, media, business, religious – began to wither. Americans didn’t know who to trust, and this lack of trust created a petri dish for conspiracies.

The age of the internet has allowed conspiracy theories to proliferate – and proliferate they have: 9/11, Pizzagate, the conspiracy theory that climate change is a hoax, which was perpetrated, Trump insists, by the Chinese. The anti-vaxxers, mired in their fears about the safety of vaccines, have precipitated a public health crisis.

A provocateur named Alex Jones has apparently fashioned an entire career out of conspiracy theories, including the doozy that the tragic shootings at Sandy Hook were staged.

And let’s not forget the conspirator-in-chief. Donald Trump likely would not be president today were it not for the so-called “birther” conspiracy, the nonsense that Obama was born in Kenya, not in the United States, and therefore was constitutionally unqualified to be president. Trump peddled “birtherism” relentlessly for years, finally prompting Obama to produce his birth certificate, the first president in history to do so.

Trump has continued to propagate conspiracy theories, most of them casting himself and his administration as victims. He thinks that the Mueller investigation was a conspiracy against him as was the investigation of his campaign’s contacts with Russian operatives, even though both probes produced copious evidence of wrongdoing.

The incessant banter about “fake news” suggests that the media is conspiring against him.

Why do so many Americans imbibe conspiracy theories? The aforementioned crisis of authority is certainly part of it, I think, but there also may be a less pernicious reason for their popularity: Some – I repeat, some – conspiracy theories can be fun, a kind of parlor game, allowing people to spin out elaborate scenarios and speculate about the actions and the motives of various individuals.

Most conspiracy theories, I hasten to add, are not that benign, especially when they threaten the nation’s democratic order. One of the more recent is something called QAnon, which insists that the so-called “deep state” is engaged in an elaborate, secret plot against Trump. (The perpetrators of QAnon participated in the invitation-only “social media summit” at the White House last month.)

Harrison Schmitt, the last man to walk on the moon, has his own theory about why Americans have become so susceptible to conspiracy theories: a failure of education. “If people decide they’re going to deny the facts of history and the facts of science and technology,” he told the New York Times, “there’s not much you can do with them.” Schmitt, who served as a Republican senator from New Mexico, added, “For most of them, I just feel sorry that we failed in their education.”

In the spirit of conspiracy, I’ll close with my own conspiracy theory, one that I’ve repeated since Election Day 2016 to anyone who will listen. Actually, it’s not so much conspiracy as an awful premonition. I fear that even if Trump is defeated in the presidential election of 2020, he will refuse to leave office. He will cry fraud and “fake news” and hunker down, insisting that he really did win re-election, all credible evidence to the contrary notwithstanding.

I have little doubt that Sean Hannity and the sycophants at Fox News would join the chorus demanding that Trump remain in office.

That, in turn, would create a constitutional crisis, especially with a lapdog Senate and a judiciary of Trump’s own making. Such a scenario might eventually involve the military, which constitutionally is under the control of the commander-in-chief.

I shudder at the prospect. And if that speculation makes me a conspiracist, I suppose I must take my place alongside the anti-fluoride cabal.

(Randall Balmer teaches at Dartmouth College.)