It feels like conspiracy theories are everywhere these days. Some people believe that vaccines are a secret plot to implant microchips in the population; others that 5G phone masts are a tool of population control. In the US, the Republican Party has been almost taken over by conspiracy theories about a stolen election. Many people believe just one theory; some people believe all of them at the same time.

When confronted with wild fantasies such as these, it’s tempting – and understandable – to dismiss anything that sounds like a “conspiracy theory” as claptrap. After all, when we use the term, we almost always mean it pejoratively. There’s an unspoken assumption that, if it’s a theory about a conspiracy, then by definition it must be bollocks.

The trouble is that – as we found, repeatedly, while writing our book Conspiracy, a history of conspiracy theories from Roman times to the present day – well, there really are conspiracies in the world.

So one of the major issues in talking about conspiracy theories is that, sometimes, they turn out to have at least a kernel of truth, and just occasionally – not often, but also not that rarely – it’s all the terribly sensible people who label them as tinfoil-hat nonsense who are wrong.

To take one famous recent example: in 2019, when a fan podcast speculated that Britney Spears was being held against her will – that she couldn’t have written this Instagram post, that she would never type : ) instead of a proper emoji, we have an anonymous source confirming all this – it would have been easy to wave it off as the product of fan obsession and overactive imaginations. But, as we’ve all learned soon enough, it turned out they were bang on.

And you don’t have to delve very deep into history – or even leave the realm of celebrity gossip – to find similar examples. If, in the 60s, you’d said that the FBI was taking time out of its busy schedule to plant malicious rumours about the actress Jean Seberg in LA Times gossip columns in an effort to ruin her life, it might have sounded like paranoid rambling. But it was completely true – Seberg was just one of the victims of the agency’s secret COINTELPRO programme of surveillance, dirty tricks and worse against perceived radicals, which most notoriously led to the state assassination of black activist Fred Hampton.

Similarly, it’s a grim fact of modern life that almost every shocking event – mass shootings, terrorist attacks – will almost instantly be dismissed by somebody, somewhere, as a “false flag”, set up or outright faked by shadowy powers to further their goals of tyranny. It has become a central pillar of many conspiracies, allowing believers to dismiss or ignore anything that doesn’t fit their views. But it really doesn’t help that, in arguing against this pernicious belief, you have to deal with the verifiable historical fact that, for example, the CIA discussed plans to do just that as part of a disinformation campaign against Cuba. OK, it never carried them out, but still.

A weird thing about all this is that, even when everybody agrees there really was a conspiracy, we’ll still come up with alternative conspiracy theories. The assassination of Abraham Lincoln was, unambiguously, the result of a group of people conspiring. The conspirators were identified and apprehended quickly – unsurprisingly, given that the man who pulled the trigger was one of the country’s most famous actors, who shot the president in a theatre where he regularly performed.

It was a comically un-covert plot.

And yet, the existence of a pretty undeniable conspiracy fact didn’t stop a flood of alternative theories – people who weren’t satisfied by the plain realities of the actual explanation, and wanted a more compelling narrative that better fit their political beliefs.

In reality, the death of almost any prominent politician will result in conspiracy theories about how they were murdered, even when there’s a perfectly simple explanation. In part, that’s because mundane deaths and boring explanations just don’t feel proportional to the trauma such deaths can cause.

But then again, perhaps it’s because assassination really has been a remarkably common cause of death for national leaders throughout history. Four out of 45 US presidents were shot dead; one study of European monarchs across 800 years of history found that close to 15 per cent of them were assassinated. When a job has around a 1 in 10 chance of ending in murder, it’s hardly surprising if people raise an eyebrow at any “unfortunate accidents”.

The assassination of a politician is, at least, a fairly straightforward kind of conspiracy. Compared with the baroque and convoluted web of interlinked plots that make up modern conspiracy beliefs such as those shared by QAnon, the idea of a conspiracy whose only goal is to knock off a president can seem almost quaint.

You can, perhaps, blame the Cold War era for at least some of this – as it included both an awful lot of conspiracy theories, and an awful lot of conspiracies. One thing you see repeatedly in the history of conspiracism is that, when people believe there’s a conspiracy against them, they’ll use that to justify operating conspiracies of their own. And so the Cold War’s heady stew of full-spectrum paranoia and seemingly existential ideological battles, with concealed enemies supposedly lurking in every shadow, gave true believers a licence to indulge their deepest conspiratorial desires.

And – as will be familiar to anyone who’s read Jon Ronson’s book about the US Army’s explorations of the paranormal, The Men Who Stare At Goats – many of these schemes were profoundly unhinged, from illegal mind-control experiments to attempts to use psychics for espionage. Honestly, it makes it a bit tricky to respond to anything but the wildest modern conspiracy theories with an “oh come on, stuff like that just doesn’t happen.”

Of course, acknowledging the existence of real conspiracies isn’t the same as believing they’re an answer to everything. A kernel of truth does not vindicate the whole nut, and the fact that there really is a secret military base called Area 51 doesn’t mean that the government is covering up aliens there (probably).

Everything we know about actual conspiracies tells us that they’re not the all-powerful villains’ plots of Hollywood scripts. Instead they’re very often small, petty, stupid and – importantly – deeply incompetent. There’s a big difference between being open to the possibility that a particular event might have been the result of people plotting behind the scenes, and the modern conspiracist mindset, in which the whole world is the product of a vast pyramid of conspiracies, and (in the words of the political scientist Michael Barkun) “everything is connected, and nothing is as it seems”.

That mindset can do profound damage, to both its believers and those caught in the crossfire. Those who fall prey to it can drive violence and murder, fuel racism and xenophobia, hamper public health and tear families apart. So it’s important to take conspiracism seriously, and counter it wherever we can.

But in doing so, we should probably take a moment to acknowledge that sometimes – just sometimes – there really might have been a conspiracy.

Conspiracy: A History of Boll*cks Theories and How Not to Fall For Them by Tom Phillips and Jonn Elledge (Headline, £16.99) is out on 7 July