In the weeks and months after the attacks of September 11, 2001, there was an image that circulated heavily online, mostly via email. It showed a man who appeared to be standing on the observation deck of one of the World Trade Center towers. Pictured over his right shoulder was the nose of a jet. A tourist had captured a photo of one of the highjacked airplanes moments before it struck the tower, the story went, and the camera found in the debris at Ground Zero was all that remained.

The photo was doctored, a digital joke made by the man pictured. It was also one of the first in a wave of widely shared urban legends and hoaxes about the attacks that have rippled across the web since 2001. Over the years, 9/11 has remained a focal point for conspiracy theorists. “The bigger the event is, the more conspiracy theories that are going to surround it,” said Joseph Uscinski, a co-author of the book American Conspiracy Theories. “With the JFK assassination, you have conspiracy theories lingering forever. 9/11 is going to be the same thing."

A 12-minute video that’s making the rounds this week claims the 1985 film Back to the Future contains a coded message warning of the 9/11 attacks. The gist of this theory: Twin Pines mall, where one of the movie's main characters is attacked by terrorists, is meant to represent the Twin Towers. (There's also something about how film is a portal to transcendence.) The name of the group that made the video, Apophenia Productions, seems appropriate. Apophenia refers to the tendency to perceive a pattern among unrelated or random ideas or objects.

“In statistics, a problem akin to apophenia is a Type I error, or false positive,” wrote Katy Waldman in a 2014 Slate article about the phenomenon. “It means believing something is real when it isn’t, based on a misleading pattern in the data.”

Truthers, the nickname often used for 9/11 conspiracy theorists, have routinely looked to Hollywood for hints that the terrorist attacks were foretold. Along with Back to the Future, some of the films that conspiracy theorists say contain warnings include: Armageddon (1998), The Matrix (1999), Gremlins 2 (1990), Rugrats in Paris (2000), Godzilla (1998), Terminator 2 (1991), Super Mario Bros. (1993), The Patriot (2000), Traffic (2000), and several others. The so-called clues, in most cases, are not compelling or even particularly eerie. Some of the films are flagged simply for referencing the numbers 9 and 11 in close proximity—on the hands of a watch, for example, or in dialogue about the birth of a 9-pound-11-ounce baby.

The fact that the people looking for conspiracies find them in Hollywood films is a reflection of the people, not the films—or even the larger prevalence of such theories. “There’s also a psychological component that underlies it,” Uscinski said, “an environmental effect that essentially triggers something that’s already there. Some people are just predisposed to view conspiracies lurking behind every corner.

“So everything that happens, I guarantee, you go to the Internet and find people who believe there’s a conspiracy about it,” Uscinski said. “Whether it was Sandy Hook or Aurora or the Deepwater Horizon oil spill. Everything that’s big in the news, people who have a conspiracy viewpoint will view it as the product of a conspiracy.”

When you start looking for conspiracy theories online, they seem to be everywhere. Dave Matthews, Nostradamus, Donald Trump, and the typeface Wingdings all predicted 9/11, according to various websites. Elsewhere, you’ll find reports that America is on the brink of a second civil war, and that Elvis and Michael Jackson are still alive.

Fundamentally, conspiracy theories haven’t actually changed much over the course of hundreds of years. “Ubiquitous access to all sorts of information is making at least some of us seem less, rather than more, rational and tolerant," wrote Carl Jensen and Rom Harré in the book, Beyond Rationality. "The problem, however, is certainly not in the information, which is agnostic. It is more likely a result of the filters we construct and those that are constructed for us.”

To track conspiracy theories over time, Uscinski and his co-author Joseph Parent scoured a century’s worth of letters to the editor of The New York Times. Among some of the themes they found were the beliefs that American scientists were controlling the weather (1958), the Roosevelt administration was secretly communist (1936), England and Canada were conspiring to overthrow portions of the United States (1890), and a bill written to protect sheep was passed with the intention of killing off all domestic dogs (1917).

These and other conspiracy theories, the researchers argue, have always been prompted by perceived asymmetries in power structures. “The causes of conspiracy theories are not primarily philosophical, psychological, or sociological—they are political,” they wrote in their book.

And if you look back over time, the same political themes emerge across multiple conspiracy theories. “[They] are marked by a distinct thematic configuration, narrative structure, and explanatory logic, as well as by the stubborn presence of a number of common motifs and tropes,” wrote Jovan Byford in Conspiracy Theories: A Critical Introduction. “Conspiracist interpretations of the 2008 financial crisis draw on the same armory of arguments and tropes which were used to interpret the Great Depression of the 1930s. The 9/11 Truther movement draws extensively on the interpretative framework established in the 1940s, when the opponents of Franklin D. Roosevelt accused him of allowing Pearl Harbor to happen in order to create a pretext for taking America to war.”

But the Internet has changed the packaging and distribution of conspiracy theories in ways that weren’t previously possible. With the democratization of publishing power, more people are more easily able to share fringe ideas across wider networks and in a variety of formats. For example, YouTube is a hugely popular platform for conspiracy theorists, who make good use of ominous music, dramatic close-ups, and slow-motion replays.

It should be noted, too, that there really are people who believe this stuff.

A CNN poll last month found 20 percent of Americans believe President Barack Obama was born outside of the United States and 29 percent of Americans believe he is a Muslim. (Obama is a Hawaii-born Christian.) Others have claimed that he secretly visited Mars as a teenager, that he’s a lizard, and that he’s the antichrist. Absurd as these theories may be, they’ve emerged at a time when conspiracy theories are, scholars argue, actually less fervent than they once were.

“Our research found every age complaining that it had reached a new peak of conspiracy theories, yet our data showed they were almost all wrong,” Parent told me. “Anyone motivated to see a pattern can cherrypick conspicuous conspiracy theories, but without a representative, long-term sample, one can’t say much about ebb and flow. That would be like picking three stocks as an index for the stock market. We find that the true peaks of American conspiracy theories were in the 1890s and 1950s, but overall they have been coming down for decades.”

In the 1890s, antisemitic conspiracy theories about Jewish economic influence were among the most prominent, and some of the narratives that emerged at the time are linger in stereotypes today. Masonic conspiracy theories, concerned with the political influence of Freemasonry, were similarly high-profile. “The belief in freemason conspiracies was huge in the 19th century,” Uscinski said. “You can go back in history and you find massive conspiracy waves, a massive outbreak of conspiracy theorizing, but those times are long in the past.”

By the 1950s, conspiracy theorists had largely turned their attention to communism. And a decade later, the Kennedy assassination and moon landing both generated enormous conspiratorial buzz. Skepticism of the official account of President John F. Kennedy’s death has, of course, persisted.

One of the more ominous characters visible on the Zapruder film, a home movie which captured footage of the president's assassination in 1963, is known as the umbrella man. Standing along the motorcade route beneath a dark umbrella on a sunny day, this mystery man became an obsession among conspiracy theorists. (He was also the subject of a conference for conspiracy buffs as recently as the 1990s.) Was the umbrella secretly a weapon? Was it a tool for signaling to a gunman? Who was this guy?

He was, apparently, Louie Steven Witt, who explained his actions in a hearing before the House Select Committee on Assassinations in 1978. The umbrella was an obscure form of protest—specifically, a reference to appeasement, the foreign policy approach under British Prime Minister Neville Chamberlain on the eve of World War II. Kennedy’s father, Joseph Kennedy, had been a Chamberlain supporter. “If the Guinness Book of World Records had a category for people who were at the wrong place at the wrong time, doing the wrong thing, I would be No. 1 in that position, without even a close runner-up,” Witt said according to several transcripts of the hearing.

There is a lesson here, says Josiah Thompson, the author of 6 Seconds in Dallas, in a 2011 interview with the filmmaker Errol Morris. “If you have any fact which you think is really sinister—is really obviously a fact which can only point to some sinister underpinning,” Thompson said. “Hey, forget it, man. Because you can never, on your own, think up all the non-sinister, perfectly valid explanations for that fact.”

If you look at any moment in history closely enough, context falls away and things stop making sense. “We wonder whether a genuine mystery is being concealed here or whether any similar scrutiny of a minute section of time and space would yield similar strangeness—gaps, inconsistencies, warps, and bubbles in the surface of circumstance,” John Updike wrote of the Umbrella Man for the New Yorker in 1967. “Perhaps, with the elements of matter, investigation passes a threshold of common sense and enters a sub-atomic realm where laws are mocked, where persons have the life-span of beta particles and the transparency of neutrinos, and where a rough kind of averaging out must substitute for the absolute truth. The truth about those seconds in Dallas is especially elusive; the search for it seems to demonstrate how perilously empiricism verges on magic.”

Magic, or something else that can’t be explained. It makes sense that conspiracy theories seem to cluster around the events that are most unbelievable as they actually happened. Conspiracy theories may be about challenging established power structures, but they’re a way of rejecting events that are too horrible to accept. “Conspiracy theorizing is an inherent part of human nature,” Uscinski said. “It’s a constant across time and it's a constant across space.”

Uscinski declines to say which, if any, conspiracy theories he believes. “I don't discount any conspiracy theory,” he told me. “I think every single one has a better than zero percent probability of being true. And each of us has to assign our own probability. I think there’s a less than 1 percent chance that we’re ruled by reptilian overlords.”