On the night of January 7 this year, hours after the terrorist attack on the Paris offices of satirical newspaper Charlie Hebdo, a senior French police officer shut himself in his office and shot himself in the head.

The French media reported that Helric Fredou, the 45-year-old deputy director of police in Limoges, a town 350 kilometres to the south of Paris, was single, childless and depressed. His death was a melancholic but irrelevant footnote to a black day on which 11 people were murdered.

To the denizens of the digital world of conspiracy theorists, however, his suicide was something else entirely. Fredou was not, as countless conspiracy websites have suggested, “one of the lead investigators” on the case. Nevertheless, to them it was obvious that he had been “suicided” – because he knew the truth.

That “truth” is that the Charlie Hebdo attack was a “false flag” operation, carried out not by two radicalised Muslim brothers but by agents of Mossad, the Israeli intelligence agency, or the CIA.

Within minutes of the news of a foiled attack on a train in France last Saturday, websites such as Project Avalon, Blacklisted News and Crimes of Empire were confidently declaring the incident a “false flag” operation that had gone wrong.

As with Charlie Hebdo and the French train attack, so with aliens at Roswell, the multiple assassins of John F Kennedy, the faking of the Moon landings, the creation of HIV by the CIA and, of course, 9/11 – the ultimate “false flag” operation.

Welcome to the evidence-lite world of the conspiracy theorist, where mysterious forces are at work and nothing is as it seems.

We all love a good conspiracy. Homeland, 24, American Odyssey – even The X-Files is returning to our screens. To most of us, conspiracy theories are just harmless fun. But there is a dark side to them, as governments around the world are beginning to recognise as they struggle to stem the rising tide of destabilising propaganda threatening to carry off society’s more credulous members.

In the borderless world of social media and smartphones, Jonathan Swift’s observation that “falsehood flies and truth comes limping after it” has never been more true.

Last month, the British prime minister David Cameron attacked the “ludicrous conspiracy theories of the extremists” that were enticing young British Muslims onto the path to radicalisation.

The Arab world has no shortage of conspiracy theories. Unsurprisingly, perhaps, many feature Israel or the United States as the sinister puppetmasters behind many of the region’s woes.

For example, Coca-Cola maintains a “Middle East rumors” section on its website, to counter often-repeated claims that its beverages contain ingredients unsuitable for Muslims and that anti-Islamic messages are hidden in its trademark logo.

Middle East governments are not above muddying the waters. In 2010, for example, Egyptian officials suggested that Mossad agents might somehow be behind a series of shark attacks on tourists in the waters off Sharm El Sheikh.

In August last year, the US ambassador was summoned to the Lebanese foreign ministry to explain false allegations that former US secretary of state Hillary Clinton had conspired with the Muslim Brotherhood to create the terrorist organisation ISIL. Gebran Bassil, Lebanon’s foreign minister, stoked the flames of speculation higher by tweeting details of the meeting to his 65,000 followers.

Psychologists say different people have different reasons for being susceptible to conspiracy theories. Personal circumstances certainly play a part, says Michael Wood, a Canadian lecturer in psychology at the University of Winchester in the United Kingdom, who specialises in the psychology of conspiracy theories. “If you feel like you’re not in control [of your life], you are more likely to believe them.”

In September 1999, exactly two years before 9/11, the journal Political Psychology carried a study by researchers at New Mexico State University, who concluded that “beliefs in conspiracies are related to feelings of alienation, powerlessness, hostility, and being disadvantaged”.

In these circumstances, says Wood, the brain faces two psychological possibilities: “Either somebody else is controlling what’s going on, or nobody is.”

People are more likely to believe that someone else is controlling what’s happening, he says, “because then, at least in principle, the world is a knowable and controllable place, and not random. Someone is in control, even if it’s not the right person.”

Azeem Ibrahim, a British Muslim who is a research professor at the US Army War College’s Strategic Studies Institute, believes that the widespread belief in conspiracy theories in the Arab world is a substitute for people taking responsibility for their own destinies.

Needless to say, having expressed this view on Al Arabiya, Ibrahim is now the subject of his very own conspiracy theory in which he stars as (you guessed it) an agent for the CIA.

“Conspiracy theories are a serious problem in the Muslim world,” says Ibrahim. “The British prime minister is absolutely right ... conspiracy theories are an integral part of the radicalisation process.”

And, he says, they are “a symptom of intellectual laziness. It is very easy to blame outside powers for all your problems, because that alleviates your responsibility for doing anything about them, your own corruption and your own communities”.

But for political scientist Matthew Gray, an associate professor at the Australian National University’s Centre for Arab and Islamic Studies, Canberra, that view ignores the wider political and historical contexts of the Arab world, where the causes of conspiracy theories go “beyond the basic or pathological explanations”.

Anti-western and anti-modern views, he wrote in his 2010 book, Conspiracy Theories in the Arab World, were “more a source than an outcome of conspiracism”. In a region undergoing radical changes, there was “a contest between traditional values and the changes brought by modernity”. Such an environment, “where earlier political ideologies have become redundant or stale ... and subsequent ideologies have been inadequate to the task of filling the void”, was a fertile breeding ground for conspiracy theories.

For a western government such as Cameron’s to tackle such theories head-on, he says, would be to play into the conspiracists’ hands. “If you hire some moderate Muslim leaders to go out and spread the word about moderation and peace in Islam, the conspiracy theorists will just say: ‘See? The West is co-opting these people and turning them against the true religion. The only answer is our more radical interpretation’.”

While no quick fix, be believes the best solution probably lies in a return to absolute transparency.

“The US government has probably been less transparent in the past 14 years than it was in the post-war period up to 2001,” he says. “The argument now is that it should be returning to as much transparency as it can – shining a light into the dark corners where conspiracy theories thrive.”

For western governments to concentrate solely on shutting down extremist Islamic websites peddling anti-western propaganda is to overlook the enemy within – the home-grown conspiracy theorists for whom their own leaders can do no right.

In the US, the events of 9/11 triggered fantastic speculation about “what really happened” on the day that 19 terrorists hijacked four aircrafts and killed 2,996 people – speculation that, as the 14th anniversary of that day approaches, shows no sign of abating.

The common, wholly unproven, theme among the various organisations searching “for 9/11 Truth” is that the American government either carried out or condoned the attacks to justify invading Muslim countries.

All three of the buildings that fell in New York say groups such as Architects and Engineers for 9/11 Truth were felled not by the impact of the aircraft, or by the resultant jet-fuel fires, but by controlled-demolition explosives, secretly planted in all three of the buildings that fell.

Other “truthers” even believe no aircraft were involved in the attacks. The world and its media were fooled, either by digitally manipulated footage or by missiles equipped with holograms that made them look like aircrafts.

Consider, for a moment, the impossibility of keeping such a complex and widespread conspiracy secret. And then dismiss it. The lack of leaks and moles, truthers will declare triumphantly, shows only how well the conspiracy is working.

It’s one thing for someone living in Gaza to believe such theories. But what is the attraction of this stuff to the comfortably well-off lawyers, pilots and engineers, living free and without much obvious oppression in the US, who are at the core of the 9/11 truth movement?

Perhaps the US establishment has shown itself to be too untrustworthy, too many times. Watergate, Bay of Pigs and – a favourite touchstone on conspiracy websites – Operation Northwoods, a scheme cooked up by senior US military chiefs in the Sixties to justify invading Cuba by covertly carrying out acts of terror on US soil and blaming Castro. JFK rejected the plan.

But whatever the possible logical justification for fearing the worst about one’s own government, psychologists say certain people are simply hardwired to assume conspiracy is afoot.

A recent paper published in the journal Social Psychological and Personality Science concluded that political extremists of any kind, wedded to “simple political solutions to societal problems”, were attracted to conspiracy beliefs. And according to a 2008 paper by psychologists at the University of Texas, some types of people are just mentally predisposed to “signal detection” – spotting “illusory patterns” where none exist, “including seeing images in noise, forming illusory correlations in stock-market information [and] perceiving conspiracies”.

Of course, none of the three key organisations – Architects and Engineers for 9/11 Truth (AE911truth), 911Truth.org and Pilots for 911 Truth – accepts it is peddling conspiracy theories.

AE911truth “doesn’t have theories”, says Andrew Steele, the group’s public relations manager, “but rather evidence that supports the need for a new investigation ... supported by those with the professional expertise to know that the US government’s own theories are unsound”.

Likewise, Mike Berger, media co-ordinator for 911Truth.org, says his organisation is merely filling the vacuum created by “the inability of journalists” to ask “the tough questions demanded by the absurd fabrications presented by the government as fact”.

All three groups also indignantly reject the suggestion that their ongoing campaigns help to fuel hatred of the West and the radicalisation of some young Muslims.

It is “easy, simplistic and convenient to blame conspiracy theories for fomenting Muslim violence”, says Berger. In fact, “the West’s persistent hypocrisy provides more power to foster Muslim rage and undermine the credibility of America and her allies”.

Closer to home, for Cameron, is British 9/11 conspiracy theorist Michael Aydinian, whose website features a series of angry on-camera monologues insisting 9/11 was “an inside job”.

It may be wild, unsubstantiated stuff, but it certainly has a following. On August 18, Aydinian celebrated more than a quarter of a million hits and 66,000 shares for one recent post. In an email exchange, Aydinian also rejects the suggestion that he is peddling baseless and dangerous conspiracy theories. “The only conspiracy theory,” he says, “is the official explanation, because the only people who could have committed 9/11 were dual national [sic] Zionists ... Cui Bono – Israel is the only country to gain.”

The Latin phrase cui bono – who benefits? – crops up a lot on conspiracy websites. This is the invisible glue used to stick random facts and conjecture together, and translates as “OK, we have no evidence – but come on!”.

Wherever they stand on a spectrum that ranges from suspicious at one extreme to downright deluded at the other, together the “truthers” are broadcasting a disturbing and pernicious message that is being heard loud and clear around the world.

A poll carried out by Ohio State University five years after 9/11 found that a third of Americans believed the US government was behind the attacks.

By 2013, a YouGov poll found that 50 per cent of Americans doubted the official account – 46 per cent suspected World Trade Center Building 7 had been brought down by planted explosives, as many “truthers” insist.

Scepticism is similarly rife in the Middle East. In 2011, a decade after the attacks, a poll by the non-partisan Pew Research Center in Washington, found that the majority of Muslims in seven Middle Eastern countries did not believe Arabs had been involved in 9/11. Scepticism was strongest in Egypt (75 per cent), Turkey (73 per cent) and the Palestinian Territories (68 per cent).

In fact, the Pew Center reported that there was “no Muslim public in which even 30 per cent accept that Arabs conducted the attacks”.

For some who believe in such shadowy plots against them, says Ibrahim, the next logical step is to support those who are taking up arms, apparently in defence of Islam.

And that, he says, is where the dangerous weapon that is the conspiracy theory comes full circle and stabs its believers in the back.

“The number one killer of Muslims around the world is neither the Americans nor the Israelis. It is other Muslims.

“But once you subscribe to these conspiracy theories, all of this is ‘justifiable’. Everybody, even other Muslims, become part of this imagined plot to subjugate Muslims and to enslave them.”

Jonathan Gornall is a freelance journalist based in London.