Why would young British men go off to risk their lives fighting jihad in the Middle East, and perhaps then return home to commit atrocities here? A consensus has emerged that they have been "radicalised". This is understood to involve a kind of brainwashing whereby impressionable young men are led astray by malicious manipulators.

Many parents unable to understand why their sons have gone to fight overseas buy into this explanation. Many have echoed the words of the father of 18-year-old Ali Kalantar from Coventry, who said: "Maybe somebody brainwashed him, because he was not like that." Many in the media perpetuate the myth, with Evan Davis on Radio 4's Today programme, for instance, asking how young Britons become "captured" by extremism.

This is at once both a terrifying and reassuring narrative. What is terrifying is the idea that anyone could have their free will neutralised by nefarious agents of evil. But what is reassuring is that this means these young men have not freely chosen their path, for reasons they believe to be good. This reassurance, however, is false. Radicalisation is not brainwashing and we cannot counter it if we pretend it is.

Brainwashing is changing someone's beliefs against their will. It is a well-worn trope of dystopian fiction, from Nineteen Eighty-Four to A Clockwork Orange, but it is arguable whether or not such a feat is actually possible in the real world. Even if it were, it would require a great deal of time and effort. Supposed examples involve US soldiers subjected to prolonged, intense mistreatment in Korean prisoner of war camps, or people living entirely within closed cults.

This is nothing like the circumstances that led British men to fight in Syria. Aseel Muthana, for example, insisted to ITV News that he didn't discuss his plans with imams at his local mosque in Cardiff or his parents because "we knew it would [have] brought us trouble". Similarly, the father of Abdullah Deghayes, a jihadi from Brighton, insisted his son and his associates were not encouraged by anyone around them. "They went of their own free will," he said. "They went without taking consent from their parents."

The idea that people freely choose to do terrible things is one that we find hard to accept. Even those who deny that anyone has free will generally accept that there is a difference between voluntary and involuntary actions, and fighting jihad does not fall into the latter camp.

The problem is not a lack of free will but a more prosaic impaired decision-making. What should really frighten us about this is that the errors jihadis make are all simply versions of much more common ones.

To understand this, we need to start by accepting that even the criminally insane do things for reasons. In the case of the Yorkshire Ripper, Peter Sutcliffe, who believed God was calling him to kill prostitutes, those reasons are clearly the product of a deluded mind. But usually there is at least something plausible in the reasons people have to do wicked things.

For jihadis, the narrative is that Islam is the true faith and that it is threatened by a hostile, kafir world. Given that millions throughout history have died to defend their religions, we cannot dismiss those who do the same now as simply deranged. What's more, living in a country with a lot of anti-Islamic feeling, there is plenty of positive reinforcement for their feelings of persecution.

But of course, people's beliefs are rarely determined by good evidence and sound reasoning alone. There are all sorts of psychological biases that make us more ready to believe some things rather than others. Young British Muslims who believe they are seen as nobodies in their own country are bound to be attracted by the idea of being heroes elsewhere. And once inside the bubble of an online network dedicated to the same cause, all their pernicious beliefs are reinforced.

The rest of us are not so different. Few of us have no reason at all to believe what we do. But what persuades us is usually more a matter of personal history and social circumstance. And once persuaded, we seek confirmation, not challenge. It is hubris to assume that we could never have followed a similar path to the jihadis if we had found ourselves at a young age marginalised in an apparently hostile culture with the promise of something more meaningful elsewhere.

There is another sense in which the narrative of radicalisation is wrong. Not so long ago, many on the left would have been happy to describe their own political epiphanies as radicalisation. Che Guevara provided the template for this – the story of how the Guatemala coup radicalised him is an oft-told inspiration for all those proudly of the radical left.

Like religious conversions, these experiences are ones that transform a person's worldview, in such a way that it appears to provide a newfound moral clarity and certainty. We ought to be suspicious of all such experiences, but they are not confined to jihadis or usually assumed to be sure signs of evil.

The truth is that what we currently call radicalisation is not some sinister manipulation, but a process by which people come to freely choose a dangerously and wickedly misguided path that they nonetheless perceive to be a virtuous calling. There is nothing psychologically unique about this. The road to inhuman terror starts with all-too-human error. Our best protection against it must therefore be nothing less than promoting good habits of thought, ones that alert us not only to the full facts but to our own psychological weaknesses.