Almost half a century has passed since John F. Kennedy’s death in Dallas. The 1964 Warren Commission concluded that Lee Harvey Oswald acted alone. But websites that expound upon Oswald’s CIA connections, about FBI coverups, right-wing saboteurs and Mafia involvement, continue to flourish. The Zapruder footage of the assassination has been endlessly scrutinized on Internet forums, where discussions continue about noise on the grassy knoll and the president’s movement after he was shot. Whether online activity has had as much impact as the questions left open by the Warren Commission is debatable, but by 2005 63 percent of Americans were polled believing more players than Oswald were involved in the assassination.

Forecast: Holding steady.

On Feb. 15, 2001, Fox aired a program called “Conspiracy Theory: Did We Land on the Moon?” hosted by “X Files” actor Mitch Pileggi. The program featured interviews with a series of theorists who believe that NASA faked the Apollo moon landings in the 1960s and 1970s, partly to garner Cold War prestige. The program initiated a rebirth in conjecture about the veracity of the moon-landing footage. How does the American flag seem to wave in the moon’s wind-free environment? Was it in fact a Hollywood production with Disney sponsorship, directed by Kubrick? Extensive rebuttals from NASA and third-party scientists debunking the conspiracy claims (the flag had a horizontal rod at its top, to keep it extended) haven’t been entirely effective. While polls showed only 5 to 6 percent of Americans doubted the moon landings in the 1990s, that figure leapt in the new century, with some polls showing 20 percent increases in doubt for younger Americans (the British are even more doubtful). Not even fleet-footed Buzz Aldrin, who slugged one conspiracy theorist, can knock down the rumors.

Forecast: Continued turbulence. Will rise and fall with public trust in government

The decade’s most potent conspiracy theory suggests the 9/11 attacks were either intentionally allowed to happen or were a false flag operation orchestrated by an organization with elements inside the government. Among the many, endlessly detailed theories: Controlled explosions brought down the twin towers; a missile, not a plane, hit the Pentagon; a U.S. fighter jet shot down Flight 93 over Pennsylvania. Government experts, academics and the world of science unanimously agree that it is all pernicious nonsense; and the 9/11 Commission report provides detailed explanations for why such theories could never be true. Still, as recently as 2006, a third of Americans polled believed “that federal officials assisted in the 9/11 terrorist attacks or took no action to stop them so the United States could go to war in the Middle East.”

Forecast: Lacking evidence, will fade over time.

The most recent addition on our list, the “Birther” conspiracy claims that Barack Obama is not eligible for the presidency because he was not born in the United States. This basic claim is easily refuted. If it persists, it could be because some conservative politicians (Sarah Palin, most notably) have suggest there’s credence to the story (Palin quickly backpedalled). In a March survey by the Harris Poll of 2,320 adults, 25 percent said they believed that Obama was “not born in the United States and so is not eligible to be president.” Still, with so much evidence clearly debunking this one, we think it might not take hold in the long run.

Forecast: Will continue to recede to the fringe.

According to a 2001 BBC News Online article, there are over 36,000 websites dedicated to conspiracy theories about Princess Diana’s death. Most popular among the theories is the idea that the British royal family orchestrated the 1997 Parisian tunnel car crash to kill the princess and her lover Dodi Al Fayed, because her relationship with the Egyptian was an embarrassment to the monarchy. An official inquest found the crash to have been an accident, and that the driver, Henri Paul (who also died), had been driving while intoxicated, but Dodi’s father, Mohmmed Al-Fayed, continues to call foul play.

Forecast: Continued decline, along with the royals’ influence

The granddaddy of all UFO conspiracies. The myth: An object crashed near Roswell, N.M., in the summer of 1947, strewing extraterrestrial debris and alien corpses. The U.S. military maintains that it was a high-altitude surveillance balloon that crashed, part of a classified government program. And that concession, more than anything, probably fueled the suspicion and fascination that has led to countless debunked reports and breathless claims of “alien footage.” Now, Roswell might best be viewed less as a spurious conspiracy and more a part of pop Americana. It has what Michael Barkun calls an “auspicious credibility”: If something, a report or an image, is on multiple websites, it doesn’t make it more true, but people are more inclined to entertain it as such.

Forecast: Grows into harmless fable

If there were ever a time to propound stories about dastardly oil companies, now is it. But before we were speculating about drilling practices, the conspiracy theorists had a more specific concern: that oil companies were lying about peak oil (the point at which the rate of oil production enters terminal decline) in order to keep prices high, and are actually sitting on deep reservoirs of oil. No clear evidence suggests any such reservoirs exist. According to Cary Cooper, distinguished professor of organizational psychology and health at Lancaster University’s Management School, the peak oil conspiracy is most popular when people are angry, at, say, the increasing price of gas, and looking for someone to blame.

Forecast: Will rise during energy crises

Dan Brown’s giant bestseller “The Da Vinci Code” may have been a work of fiction, but it created its own cottage industry of conspiracy theories about the Catholic Church – which, according to the “Code,” has supposedly been suppressing the feminist theology necessary for a complete understanding of early Christianity. The novel posits an organization called The Priory of Sion – a cult whose members included Isaac Newton and Leonardo Da Vinci among others – who try and preserve the truth. Fact: There was a real Priory of Sion. But unlike Brown’s ancient group, it was formed and dissolved in 1956. In Professor Cooper’s view, when a book or film – albeit fictional – offers a comprehensive narrative to support a conspiracy theory, the theory gains support.

Forecast: Decreases as bestselling vampires surge

A perennial conspiracy theory that has taken various forms over the years holds that a secret group of powerful men control economies, create wars, famines and disasters for their own purposes, using their governments as puppets. (A favorite: David Icke, a British conspiracy theorist, propounds the view that world leaders are in fact shape-shifting 7-foot-tall reptilian aliens trying to destroy the Earth.) But you can lump conspiracies of all sorts of societies, from the Illuminati, the Freemasons and the Bilderberger Group, into this category. Barkun calls these “superconspiracies,” a system of top-down control and order that can serve to make an ostensibly chaotic situation (like a financial crisis) more simple to understand.

Forecast: Will last as long as governments do

We need Google. It’s becoming our source for everything. It has pictures of our houses, it knows who our friends are. What if, as with Skynet in the dystopian “Terminator” films, Google were to become self-aware and rebel against its creators? If you think this is a possibility, you might also describe the Internet as a series of tubes, but there’s no doubt that we find something unnerving about our increasing reliance on all things Google. And there is some delightful irony too, that the newest vehicle for conspiracy theories has become the subject of theorists itself.

Forecast: As our reliance on the Goog grows, so will our fears.