I was standing in a Denver convention center the first time someone asked me to defend what seemed to me a very obvious fact. If I was so certain the Earth was round, the person asked, why didn’t I prove it?

I rattled off some easy answers: you can observe the sun setting and ships vanishing over the horizon; astronauts have photographed the globe from space.

But this person had his own rebuttals. My observations of the horizon were optical illusions, he insisted, and the outer-space photographs were actually dupes, forged by multiple governments intent on concealing the Earth’s true shape.

I don’t know what I expected. After all, I was attending the Flat Earth International Conference, an annual extravaganza for a growing movement of conspiracy theorists who believe the planet is more of a circular disc with a dome. Is this debate worth my time?, I wondered. Will it improve anything at all?

As I researched my forthcoming book, Off the Edge: Flat Earthers, Conspiracy Culture, and Why People Will Believe Anything, I learned that the answer, historically speaking, was no. Disputing conspiracy theorists can feel tempting, even important. But I’ve found that debates are among the least helpful tactics for deprogramming believers and can lead, paradoxically, to even more committed conspiracy theorists.

Modern flat-Earth theory is mostly one man’s fault. In 1849, the British inventor and quack doctor Samuel Birley Rowbotham began selling his self-published 16-page pamphlet Zetetic Astronomy: Earth Not a Globe. Rowbotham, under the pseudonym “Parallax,” argued that Earth was not spherical but flat, with the North Pole at the center of the disc and Antarctica spread like a frozen ring around Earth’s outer rim.

Rowbotham and his theory became notorious when he went on a lecture circuit, where he encouraged people to debate him. Few were successful—Rowbotham was quick-witted and persuasive, and he often succeeded in winning new converts to the theory.

One of his most devoted followers was William Carpenter, a printing-press owner. Carpenter, who loved Rowbotham’s idea more for its poke to the scientific establishment than for biblical fealty, moved with his wife and six children in 1879 to Baltimore, where he taught shorthand and—determined to rid the world of round-Earth ideas—authored the scathing books Theoretical Astronomy Examined and Exposed and 100 Proofs That the Earth Is Not a Globe.

Carpenter and his independently wealthy business partner, John Hampden—a Protestant rector’s son—would eventually make history when Hampden challenged acclaimed scientist Alfred Russel Wallace to a flat-Earth debate and £500 wager. While Wallace prepared to demonstrate the Earth’s curvature, Hampden picked Carpenter as his referee and chose a stretch of land previously used by Rowbotham to prove the Earth flat. This set off a bitter, decades-long legal feud, during which Wallace demanded his money back and Hampden embarked on an extraordinary 15-year campaign of abuse and libel that landed him in both jail and court several times.

Many modern flat-earthers also attribute their introduction to the theory to a debate. Flat-earther podcaster David Weiss says he converted to the theory after trying and failing to disprove it. Robbie Davidson, the founder of the Flat Earth International Conference, told me he discovered the theory through a theological disagreement with an atheist, who referred to it in jest.

So how can conspiracy theorists be persuaded to accept other ideas? Many of the psychologists, conspiracy experts, and former flat-earthers I spoke to said respectful conversations, as opposed to combative debates, tend to more successfully change people’s minds.

One former flat-earther, who used to make YouTube videos about the theory, told me he finally came around to the globe after a pro-science YouTuber reached out, initiated a friendship, and talked him through the failings of the flat-Earth theory, point by point.

“A lot of it is about trust,” this former flat-earther told me. Today he makes videos about the globe.

Kelly Weill is a reporter for The Daily Beast. Her book, Off the Edge: Flat Earthers, Conspiracy Culture, and Why People Will Believe Anything, will be published on February 22 by Algonquin