Brainwashing is not just the stuff of B-movies and cults, it is a real threat, says Kathleen Taylor.

What do you think of when you hear the word "brainwashing"? B-movies? Newspaper stories of cults? The Manchurian Candidate (the 1962 original film or the remake, released in the UK last week)? All of these depict brainwashing as a mysterious torture, the ultimate control fantasy: victims think that they are acting freely, yet someone else is pulling the strings. As well as being a useful plot device, this "brainwashing" veils our ignorance, providing a label that sounds like an explanation. Most of all, however, it taps into one of our deepest terrors: the fear of losing control of ourselves, of being manipulated by malevolent others.

It is this fear that gives brainwashing its impact and ensures that it resurfaces whenever we need an explanation for certain peculiarly abhorrent human actions. The word was widely used in the 1950s and early 1960s. It resurfaced in the late 1960s as the Manson family dominated the headlines, was evoked in the mid-1970s by heiress-turned-terrorist Patty Hearst and in 1978 after the mass murder suicide in the supposedly utopian community of Jonestown. More recently, there have been cults - and, of course, 9/11. In all these cases, the general feeling is that something strange must have happened to make healthy people commit such terrible and self-destructive atrocities.

What happened was certainly strange. But it was not the evil magic depicted in movies. No switches flipped to activate a brain implant, no neurosurgical procedures mangled people's neurons. Instead, mind manipulators such as Charles Manson and Jim Jones used increasingly well-understood psychological processes to achieve extreme versions of an everyday occurrence. We influence each other's minds all the time. We transmit our beliefs in language and reinforce the words with gestures and body language. Most of these attempts at influencing others' actions and thoughts are brief and don't involve coercion, even when the emotional temperature rises. They are also explicit and consensual. The target is well aware that someone is trying to change his or her mind and can accept or reject the proposed belief.

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Sometimes, however, social influences are less transparent. Consider advertising and the media. The adverts that make up so much of the noise we now live in are, of course, honest about their intentions: they want to sell products. But they also transmit deeper messages that reinforce beliefs you may not have even realised you hold. People in adverts are rarely disabled, elderly, gay, overweight or unattractive; they have kitchens to die for and they never pick their noses. They show us that money brings happiness and that certain body shapes, families, behaviours and even ages are valued over others. The same is true of the assumptions that frame our news stories. Sick or dead children are innocents. Science is all about breakthroughs. Terrorists are irrational, and they can strike anywhere, any time, while being predictable enough for our leaders to be able to defeat them - whatever "defeat" means. This is not the real world as we know it, and people who take their knowledge uncritically from these sources tend to have correspondingly unrealistic views.

But deceit is not the only way to influence someone's beliefs. Sometimes the emotional temperature soars so high that we can no longer hang on to our ability to make rational decisions. Or the influencer may simply refuse to take no for an answer. Sometimes that person can control a victim's world to such an extent that isolation, repetition, emotional overload and physical abuse can be used to break down their resistance. These are the extreme cases we think of as "classic" brainwashing.

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As with less brutal forms of social influence, brainwashing's ultimate aim is to change a person's beliefs. The technique is a simple one: control the messages that reach the target's brain. Every form of belief manipulation relies on this approach, and on three vital features of human brains.

First, brains get their beliefs from the world around them and construct a model of that world by merging the information available at any given time with the information they have stored in memory. Second, beliefs are represented in the patterns of connections between brain cells, cells that are activated when signals reach them from other cells, from the body (providing the signals we call emotions), or from the external world.

Third, these connections change their strength - the ease with which they allow information to flow from one cell to another - depending on how often those neurons have been active in the past. So beliefs we don't care much about or don't think about often tend to fade, while beliefs that we give a lot of time to, or that matter to us, tend to get stronger.

This is why isolation, repetition, emotional overload and physical abuse are used in brainwashing. Isolation reduces the variety of competing information sources that might activate conflicting beliefs and thereby weaken the attempt at influencing thought patterns: cult members tend not to read newspapers or talk to people with alternative points of view. Repetition reactivates the same old circuits time and again, boosting beliefs by strengthening neural connections. (And repetition can be a seriously potent weapon: in cults, for instance, a leader may lecture his followers for hours at a time, interspersing this with discussion groups, self-criticism sessions and so on, so that the same messages are reiterated almost continuously, bar sleep, for weeks or months on end.) Emotional overload floods the brain with stress signals, drowning out the voices of memory that might otherwise object to the new dogma. Stress is also unpleasant, demanding action to relieve it - any action, even if that means renouncing one's previous existence. Brainwashers always give their victims a clear escape route: "accept my message". Finally, physical abuse, including inadequate diet and sleep deprivation, saps the energies that we need to think straight. Even in normal circumstances, consciously reflecting on the messages we receive is hard work, which is why we often don't bother. After all, the brain evolved to be lazy. Lazy, stereotypical thinking saves time and effort and leads to faster actions - that may save one's life in a crisis. Selection pressures on early Homo sapiens were concerned with the species' survival, not its susceptibility to thought control.

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These days, however, we sophisticated Westerners are less concerned with predation and reproduction. Most of us, fortunately, will never get entangled in a cult, fall for a terrorist or end up in Guantanamo Bay. But we can and do succumb to less coercive tactics, as the recent US election's focus on terrorism showed. Campaign coverage rehearsed the threat of militant Islamism while telling US voters very little about it, isolating them from other information sources and leaving them as ignorant of their new enemy as they were of its Soviet predecessor. George W. Bush's simple campaign message ("be afraid; I'll save you") evoked the very fear it purported to soothe. It was endlessly repeated (campaign spending on advertising was about $3.9 billion or ?2.1 billion). Critics who played down the terrorist threat were ignored; relevant issues such as the economy were marginalised. Uneasy voters chose the clearest, easiest message and why not? That's what brains evolved to do.

Resisting this kind of manipulation is possible. But, like brainwashing itself, it takes serious effort. Media literacy - ensuring access to several genuinely independent sources of information, preventing excessive message repetition, training people to notice emotional manipulation - certainly helps. But busy, stressed and weary people, accustomed to ignorance and heavily reliant on a tiny clique of leaders for their world-view, may not have the time and motivation to challenge their sources or search the internet for new ones. Our leaders, who gain much of their power from controlling information, have little interest in sharing that power, as the sorry tale of Labour's manifesto commitments on freedom of information shows. Even our universities, traditionally centres of free thought, seem less hospitable to different views these days. Yet we still have traditions of intellectual tolerance, scepticism, liberal freedoms and a respect for teaching people to think for themselves. Those traditions are our strongest defence against brainwashing. We must make sure, in our newly threatened world, that they are kept alive.

Kathleen Taylor is a research scientist in the physiology department, Oxford University. Her book Brainwashing: The Science of Thought Control is published this week by Oxford University Press, price ?18.99.