A conspiracy: a secret agreement between two or more persons to commit an illegal act. From the Latin con, meaning 'with', and spiro, meaning 'I breathe'. A fairly general category, then, leaving open myriad possibilities for a wide range of nefarious activities. But it's with the element of secrecy that the trouble starts. If it's done in secret, how do you ever prove it? This gives us the conspiracy theory. Because if a conspiracy is proven it becomes a fact and enters the log book of history. If it remains unproven, on the other hand - merely a theory - it will soon fall prey to the accumulating and corrosive powers of doubt, of indredulity and, ultimately, of ridicule.

David Aaronovitch, in his 2009 book Voodoo Histories, defines a conspiracy theory as 'the attribution of deliberate agency to something that is more likely to be accidental or unintended.' He refines this further as, 'the unnecessary assumption of conspiracy when other explanations are more probable'.

All you need is a quick look at the greatest hits of the conspiracy-theory back catalogue - Pearl Harbour, JFK, RFK, the moon landings, the death of Diana, 9/11 - to see the extent of the problem. Each of these cases on its own (with the single exception, let's face it, of the moon landings) is a tantalizing mystery that can easily pull you in and seduce you with unexplained 'facts' and plausible retrofits. Put them all together, however, and multiply them by the internet, and you'll just as quickly end up feeling gorged and slightly ashamed of yourself for having even started down this path in the first place.

John Lahr explains it as, 'mankind's compulsive need for narrative to alleviate the anxiety of not-knowing.' Which leads us to the conspiracy thriller, an altogether different beast from the conspiracy theory. What the (good) thriller does - in contrast to the theory, which harangues and proselytizes - is put real people into the equation. It then tells a story and by so doing attempts to alleviate the rather different anxiety of not-understanding.

Set against a dark mood of suspicion and disillusionment, the great conspiracy thrillers of the 1970s charted the gradual alienation and disempowerment of the individual in modern society - the stripping away of privacy and the growing influence of shadowy governmental and corporate structures. But it was really the central characters in these stories that gave them such enduring power - John Klute, for example, in Klute, Joe Frady in The Parallax View, Harry Caul in The Conversation. The thing is, these movies weren't really about proving a particular theory or uncovering a specific plot, they were about something bigger, a fatal loss of innocence, and one suffered by a whole generation of Americans.

In the wake of Eisenhower's warning about the military-industrial complex, and then, in succession, of the Kennedy assassinations, Vietnam and Watergate, it was no longer possible for the ordinary citizen, the little guy, to buy into the American dream as unquestioningly, as unequivocally, as he once had. It was shocking to find out that governments didn't always tell the truth and that business corporations routinely sacrificed people's lives for profit. But this was a new reality and these movies - with their almost fetishized landscapes of impersonal architecture and encroaching technology - did their best to reflect it.

After the 70s, however, something strange happened, a sort of moral fatigue set in. In the 80s and 90s, the conspiracy thriller and the conspiracy theory seemed to edge closer to one other and finally meld, producing the bizarre, baroque love child that was Oliver Stone's JFK. After that it was no giant leap to go from Donald Sutherland's Mr X to Chris Carter's The X-Files, from the Smoking Gun to the Smoking Man. Before you knew it, we were paralysed by irony. Before you knew it, stories with government or corporate conspiracies in them were being dismissed as crackpot, the tropes of the form derided as lame and clichéd. In fact, as a result of this you might even have been forgiven for concluding that not only was the conspiracy thriller tired it was actually redundant - that things had changed for the better, that governments no longer spied on or lied to their citizens, that corporations no longer put the profit motive before any moral consideration of their actions, or that Deep Throat's exhortation in that underground carpark all those years ago to "follow the money" somehow, happily, didn't apply anymore.

But then 9/11 came along and the mood music changed forever. Within a couple of years it was no longer possible to ignore the truth (undeniably out there) that since the 70s there has seen a breathtaking increase - exponential, Moore's Law-like - in every form of electronic surveillance, in the influence, reach and wealth of transnational corporations, and in the sinister privatization of the military and intelligence industries. These matters were being taken seriously once again, and as a direct result, conspiracy - in both its forms - was back in business.

On the theory front we've had an embarrassment of riches - 9/11 itself (with a side order of WTC Building 7), WMD, David Kelly, peak oil and the New World Order. On the thriller front we've had corresponding riches in the form of novels (Call Me Ishmael, The Constant Gardener, A Simple Act of Violence, Truth), movies (Syriana, the Bourne trilogy, Michael Clayton) and TV shows (State of Play, Rubicon, The Shadow Line). But again, what separates the two forms is the human factor.

The conspiracy theorists strive to reshape the world in absolutist terms, the writers of conspiracy thrillers merely strive to negotiate the world as they find it. The theories tend to be all-or-nothing, one-dimensional and aggressively polemical. Each of the thrillers mentioned above is complex and ambiguous. Each contains a believable, sympathetic individual struggling to survive in a web of corporate, political or institutional corruption, and as the stories unfold their knotty emotional lives are shown to be intextricable from the wider moral context.

I'm very fond of conspiracy theories - they're intriguing and fun, in their twisted way. But they're also very frustrating, because any big reveal will always be just out of reach, always just around the next corner. You want them to be true, and maybe some day they will be, but they're never true now. Conspiracy thrillers, on the other hand, are a lot more satisfying - and the thing about them is, regardless of what they're about, regardless of what they posit, if they're honest and well-written, they're already true before you open the first page.

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