There are many sound reasons for Scots to vote yes on Thursday but a secret oilfield near Shetland is not one of them. The story began when David Cameron made a brief, low-key visit to Shetland in July, ostensibly to announce a household energy deal but in fact, say the rumours, to inspect a new oilfield at Clair Ridge so valuable that it could mean untold wealth for an independent Scotland. The theory, which went viral among nationalists, claimed that the discovery of the field was being covered up lest it sway the referendum.

It’s a bold claim but it isn’t reserved for serial conspiracy theorists. Last week, a YouGov poll of 1,084 Scots, commissioned by Buzzfeed, found that 42% believed the Clair Ridge theory was “probably true”, with 26% agreeing that MI5 was probably working with the UK government to prevent independence. Obviously, I can’t disprove the existence of the oilfield. I can say that there is no evidence for it, point out that such a plot would require a degree of ambition and cunning hitherto unglimpsed in the no campaign, and question why Cameron would have blown his cover by tweeting a photograph of himself stroking a Shetland pony. But I cannot prove it doesn’t exist. That’s the magic of conspiracy theories.

It’s no longer helpful to refer to conspiracy theorists as a distinct group of eccentrics. Rather, conspiracist thinking is a spectrum that increasingly includes many people who consider themselves well-informed and level-headed. You don’t have to search for examples; they pop up all the time on social media. The other day, someone posted a comment on a friend’s Facebook thread, suggesting that Islamic State was a creation of the CIA and that the videos of hostages being beheaded had been doctored. He was very matter-of-fact about it.

Conspiracy theories on social media usually take the form of a jpeg with a dramatic claim and a couple of unsourced and/or inconclusive facts, encouraging readers to share them and thus join an army of fearless truth-seekers outwitting the powers-that-be. You may have seen the recent one that claimed the BBC had pulled Jeremy Bowen from Gaza for being pro-Hamas, even though Bowen was simply on holiday. No wonder the Facebook category on the PolitiFact website’s Truth-O-Meter is the one most likely to earn a damning “Pants on Fire” rating.

This summer, with its smorgasbord of horrors, has sent the conspiracy theory machine into overdrive. Ukraine shot down Flight MH17 to garner sympathy and demonise Russia. Islamic State was created by a US/Zionist plot codenamed Hornet’s Nest to destabilise the region and protect Israel. Governments are planning to quarantine entire cities at gunpoint if the Ebola epidemic spreads.

There are very good reasons to be sceptical about the motives and veracity of western governments and media outlets, up to a point. But those who automatically question the BBC or CNN are remarkably credulous when it comes to other sources. At best, this means retweeting an unsourced meme without checking it’s true; at worst, it means endorsing propaganda. The wildest theories about MH17 and Islamic State didn’t originate with plucky keyboard warriors but the state medias of Russia and Iran.

In a climate of (understandable) anti-politics, anti-elitist sentiment, theories that run counter to mainstream opinion are seen as more likely to be true, not less, and no amount of data or denials can quash them. But the logic is faulty. Real scandals tend to arise from arrogance, incompetence and short-termism, not labyrinthine schemes requiring the lifelong discretion of hundreds of plotters. The likes of Noam Chomsky loudly oppose conspiracy theories because they deflect outrage that should be aimed at genuine crimes and deceptions and channel it into fantasies. These fantasies are not harmless. The groundless belief that childhood vaccinations can cause anything from allergies to autism is conspiracist thinking because it draws on a similar suspicion of conventional wisdom and implies that the authorities are, whether maliciously or recklessly, endangering children.

The anti-vaccine movement has flourished among well-educated, progressive families. In a recent report on a whooping cough outbreak in Los Angeles’ fancier schools, the Hollywood Reporter found that distrust of the drug companies and the “arrogant” medical establishment fused with the valorisation of free-thinking and alternative medicine, creating an anti-vaccine wave with dire consequences for public health. In his book about the vaccine controversy, The Panic Virus, journalist Seth Mnookin bemoans “the notion that our feelings are a more reliable barometer of reality than the facts”.

Feelings before facts is the fundamental operating principle of conspiracy theories. The most diligent theorists have reams of data to support their case but it has been selected by confirmation bias and the belief is too strong to be changed by contradictory information. Once a believer has found enough like-minded souls, whether at the gates of a Hollywood nursery or on a Facebook thread, they have created a self-affirming bubble that the truth can’t penetrate.

For anyone who hasn’t gone past the point of no return, the only antidote is to be as sceptical of maverick sources as you are of official ones, and to spend a couple of minutes checking the facts before you post a seductively dramatic meme that feels correct just because it confirms your prejudices. Conspiracy theories pose as a path to the truth. More often, they are a roadblock.