On 12 January 2009, a hardcore of protesters broke away from a peaceful march in London to vandalise a Starbucks in the East End. Five days later, following a rally in Trafalgar Square, another two Starbucks were ransacked and looted. Nothing unusual there, you may think. After all, along with McDonald's and high street banks, the US coffee chain is an obvious target for anti-capitalists. These London attacks, however, were not spurred by a big business backlash. They were inspired by something far more sinister: conspiracy theory.

The vandalism followed protests against Israeli actions in Palestine and Gaza, and Starbucks was targeted largely because the company, whose CEO Howard Schultz happens to be Jewish, has been the subject of several long-running conspiracies which range from the belief that the company donates profits to the Israeli military, to the suggestion that the woman on Starbucks' logo is Esther, a biblical Jewish prophet.

Starbucks flatly denies these allegations and has taken great pains to identify its sources, pointing out that the logo myth came about because an unintentional similarity between a children's book cover drawing of Queen Esther and the firm's logo, which is actually derived from a 16th-century Norse woodcut. But the assertion of fact largely fell on deaf ears. A few days after the attacks on the London outlets, Egyptian cleric Safwat Higazi went on television in his homeland to reinforce the logo myth.

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"The girl in the Starbucks logo is Queen Esther," he ranted. "This queen is Queen of the Jews. Can you believe that in Mecca, Medina, Cairo, Damascus, Kuwait and all over the Islamic world hangs the picture of Queen Esther and we buy her products? It is inconceivable!"

The military financing theory is equally traceable. It grew from a spoof letter written in 2006 by Australian anti-Semite Andrew Winkler. The letter regularly surfaces to fuel anti-Israeli feeling. It was circulated via e-mail and social networks in 2010 a few days after nine people were killed by Israeli troops raiding a Gaza-bound Turkish aid flotilla. The viral message spurred a protest outside one of Cairo's Starbucks branches and the company urged people in the region "to verify the facts about Starbucks from reputable and respected sources and to share those facts".

Conspiracy theories are cultural viruses. Once they infect the zeitgeist, it is extremely difficult to stamp them out – no matter how solid the evidence against them is. Studies have shown that people who are prone to believe in conspiracies display an innate bias towards information which supports that conspiracy, no matter how spurious that information is and no matter how solid the evidence against the conspiracy is.

Today, there are more conspiracy theories and more conspiracy theory believers than ever before. They range from the simply fanciful – such as the theory that Kentucky Fried Chicken is owned by the Ku Klux Klan which laces the food with a drug that makes only black men impotent – to the labyrinthine, such as the intricacy of theories around 9/11 and the death of John F Kennedy.

Almost every world event spurns a set of conspiracies. According to one new theory, Muammar Gaddafi was not overthrown because he was a crazed brutal dictator; he was ousted and killed because he was plotting to introduce a new Africa-wide trading currency to threaten the dollar. Gaddafi himself was an arch conspiracist. Early on in Libya's uprising, he blamed Osama bin Laden for influencing the rebellion and claimed rebels were fuelled into action by LSD. Perhaps fittingly, there are now conspiracies which claim his death photographs are faked.

From terrorist attacks to high-profile deaths such as those of Princess Diana, bin Laden, and Michael Jackson, each major news event spawns a set of suspicions. While some are harmless, others, such as the Starbucks conspiracy, are used to bolster extremist ideologies. Easily spread through the internet, they can incite and inspire misguided actions and, at their most dangerous, they provide cohesion for terrorist groups, unifying followers behind a particular cause or against a perceived enemy.

Once the quaint preserve of anoraks (think JFK) and X Files fans (alien abduction, faked moon landings and Area 51), they have now become a malevolent modern-day tool which nefarious organisations use to further their aims. Jamie Bartlett, head of the Violence and Extremism Programme at independent think tank Demos, has studied this worrying trend.

"We looked at 50 organisations including far right, far left, cults, religious extremists, radical Christians, radical Muslims, and what we found was that every one of them has some kind of conspiracy attached to it," he says. "The members believe in a conspiracy; sometimes it is a big global one, sometimes one directed at their specific group or interest. The conspiracy holds the group together and pushes it in a more radical direction. It serves to harden the group's ideology."

There are several reasons why conspiracy theories are increasing. Mainly it is because the internet has made it easy to propagate rumour and supposition on a global scale. Social networking sites allow conspiracy theorists to seek out and link with like-minded individuals. Whereas past conspiracies, like those surrounding the death of JFK, took years to formulate and disseminate, today's conspiracies develop almost organically. Immediately after 9/11, the internet was abuzz with individual voices questioning the official version of events. These nebulous ideas were able to crystallise as theorists discussed and developed their ideas and formed into a set of theories adopted by groups such as the 9/11 Truth Movement.

People interested in conspiracy also have access to vast online depositories of reference material which can be selectively edited to support an idea. Mobiles with in-built video cameras allow footage of events to be uploaded to YouTube instantly. Video sharing and easily-edited visual content has meant those interested in conspiracies are getting younger and, as such, are a more impressionable audience. Interest in conspiracy has developed into a counter-cultural youth movement.

Bartlett explains: "You still get the old-school anorak conspiracy theorist who spends a lot of time poring over journals looking for tiny anomalies. But you also have the student types who think it is cool to be anti-government, anti-US or anti-imperialist. Then you have a large number of young people from disadvantaged and minority backgrounds who haven't looked into any evidence but accept conspiracies because they hold the belief that the government is corrupt."

The number of people who believe conspiracy theories is staggering. According to various recent surveys, a third of Brits believe Princess Diana was murdered (a Daily Mail survey), a quarter believe the moon landings were faked (from Engineering and Technology magazine), nearly half of all Americans do not believe global warming is man-made (a Yale University survey) and 84 per cent of them believe 9/11 was an inside job (a New York Times/CBS poll).

The rise of conspiracy as a cultural phenomenon can in part be attributed to the uncertain times we live in. In the same way that paranormal beliefs and religious extremism peak during times of economic and social upheaval, so too does the number of people who believe in conspiracies. Yet conspiracy theory remains a little-studied area.

Dr Karen Douglas, Associate Professor at the School of Psychology, University of Kent, is one of the leading academics in the field. At a recent conference in London, she presented her work on the psychological factors which drive people to believe in conspiracies.

"There isn't a great deal of data out there on why people believe in conspiracy theories – in terms of research it is a bit of a blank slate," she says.

Studies have identified a core set of psychological variables which correlate to belief in conspiracy theories. Perhaps, unsurprisingly, these include low levels of trust – not just in authority but in individuals – and high levels of anomie, the feeling that things are getting worse, alienation and powerlessness.

In one of her studies, Dr Douglas tested how influential conspiracy theories can be. Two randomly selected groups of people were tested to see how attitudes and belief systems are altered when people are exposed to conspiracy theory material. One group was given material to read which suggested theories about Princess Diana's death and contained details alluding to the common conspiracy that she was killed by the Establishment.

Both groups were then asked a series of questions about their belief systems and their belief in conspiracies. Respondents were asked to grade their answers. Analysis of the answers suggested that those who had read the material were more likely to believe other theories.

"What we found was that people were actually strongly influenced by the theories even though they thought they hadn't been," explains Douglas. "You don't think you are being influenced by them but you are. When you read about them all the time and they are everywhere and growing in popularity, they start to influence how you interpret significant events."

This mental contagion is reinforced even further because, according to studies, belief in one theory suggests believers will accept other unrelated theories. So if you believe Disney planted subliminal messages about sex in the movie The Lion King, you are also likely to believe mobile phone GPS technology is used by the government to monitor citizens, or that the Wingdings font included with Windows has been used to send hidden messages.

As Chris French, Professor of Psychology and Co-ordinator of Anomalistic Psychology Research Unit at Goldsmiths explains: "The single strongest predictor of whether you believe in a conspiracy theory is whether you believe in other conspiracy theories, even when there is no connection".

Along with his colleague, Robert Brotherton, French has identified a number of personality traits which correlate to whether someone will be susceptible to conspiracy beliefs. These include people who believe things happen to them, rather than as a result of their own actions, and people who hold religious beliefs.

French continues: "Believers are mainly people who are trying to make sense of a chaotic universe and looking for some form of framework to guide them to do that".

Indeed, we as human beings often need major world events to be explained by equally major causes. It is too frightening to live in a world where 19 Arabs with box-cutters can change the course of history in a matter of a few hours. Instead, it is easier to believe that a huge government conspiracy was behind 9/11.

A mistrust of government is a key factor behind many theories. The most enduring of which almost always involve some aspect of malign and underhand government or secret-service activity. While some conspiracy theories remain largely harmless to society, ones that foster mistrust in authority are increasingly being shown to have serious large-scale consequences.

One study carried out by Dr Douglas and her team at the University of Kent discovered that conspiracy theories affect the democratic process. "We asked people to read a range of theories about the government. Without any supporting evidence the theories suggested that governments hide information and are involved in shady plots and that we should be suspicious of them. We found that the people who read that kind of information were more reluctant to engage in the political processes."

The conclusions both Douglas's and French's studies have drawn are that increasingly, people are basing important decisions about issues ranging from voting to vaccinations on conspiracy theory-derived information they read on the internet.

"This is a big issue with a lot of serious implications," says French. "People ... are making life-changing decisions without employing any critical thinking skills."

This lack of critical thinking reinforces questionable belief systems and allows them to spread. Its prevalence in society led the Demos conspiracy study team to call on the Government to introduce measures in schools to help students distinguish between reliable and spurious online sources of information.

They found that schoolchildren were unable to differentiate between the two and that one third of pupils between 11 and 13 thought that Google organises its search hits in order of the reliability of content. They had no idea how search engine optimisation works.

Bartlett explains: "The national curriculum needs to get on top of this and teach kids about these things. In the information age, creation and sharing of content is more unmediated than ever before and there are no useful signs to differentiate between what is good and what is bad. There is an intrinsic value in getting to the truth of something and that is being lost. The fact that lots of people hold absurd propositions regardless of evidence is uncomfortable. It is worrying because of the way in which people are going about understanding the world."

This willingness to believe without question explains why some people have been able to make profitable livings by peddling conspiracy. Take David Icke, for example. The purple tracksuit-clad former BBC presenter and self-proclaimed "son of God" claims that humanity is actually under the control of dinosauroid-like alien reptiles who must consume human blood to maintain their human appearance. As insane as it sounds, he still manages to sell books based on his theories and to pack lecture halls.

At a recent conspiracy theory conference in London, organised by the Centre for Inquiry, and attended by sceptics and theorists alike, some professional theorists took the opportunity to hawk their wares. Ironically, some of the theorists in the audience claimed the academic data presented was part of a conspiracy to discredit conspiracies.

Although it is worth pointing out that some conspiracy theories have been proved true (Watergate, for instance, proved right suspicions about Nixon's dishonest activities) and that authority should be questioned, the warped and selective mindset displayed by die-hard political conspiracy theorists leads to a lack of understanding about how governments work and, on a psychological level, stops people reasoning properly and making deductions based on evidence.

"If you do believe in conspiracy theories, the reasoning you apply is illogical, emotion-driven, irrational, and non-evidence based," says Bartlett. "I don't like people believing nonsense because it doesn't do them any good. Conspiracy theories absolutely demolish the small modicum of trust we still have in our governments. We still need people to trust that sometimes authorities do the right thing, yet there are millions of people who genuinely believe they conspire against and murder their own people."

The truth is out there, and if you know the right places to look, it is not hard to find. But as long as conspiracy theories continue to be propagated and believed on the scale they are today, the truth will remain masked by crackpot theory and myth.

Top five wacky conspiracy theories

Reptiles rule the world

The former BBC presenter-turned-'son of God' David Icke claimed that humanity is actually under the control of dinosauroid-like alien reptiles who must consume human blood to maintain their human appearance.

The moon landings were faked

A classic among conspiracy theories: proponents allege that the moon landings never took place, and were faked by Nasa with possible CIA support.

Bar codes = mind control

Some conspiracy theorists have proposed that barcodes are really intended to serve as a means of control by a secretive world government. Others believe they contain the 'mark of the beast'.

Font of secret knowledge

The Wingdings font is said to feature subliminal messages. In 1992, it was discovered that the character sequence 'NYC' in Wingdings was a skull and crossbones symbol, a Star of David, and a thumbs-up gesture. Microsoft strongly denied this was intentional.

Finger lickin' foolery

One theory purports that the Kentucky Fried Chicken franchise is owned by the Ku Klux Klan, and that the chicken is laced with a drug that makes only black men impotent.