At the end of the Korean War, 21 American prisoners-of-war chose to remain in communist Korea and openly sided with an enemy that had killed thousands of their comrades. In addition, a surprisingly large number of the American service personnel who did return home enthusiastically expounded the strengths of communism. The family and friends of these servicemen were stunned, and the world’s media flocked to Korea to report the story. Some researchers suggested that the Koreans had brainwashed the American soldiers with flashing lights, hypnosis or mind-altering drugs.

They were all wrong.

My latest book, Rip It Up, examines the curious relationship between behaviour and thought. Your everyday experience tells you that your thoughts cause you to behave in certain ways. Feeling happy makes you smile, and feeling sad makes you frown. However, decades of research have revealed that the exact opposite is also true - behaviour creates thoughts. When you smile you feel happier, and when you frown you feel sad. The same effect applies to belief. Get people to behave as if they hold a certain belief and bingo, they start to actually believe.

Extensive interviews with prisoners-of-war who returned from Korea revealed that the Chinese authorities had employed this principle.

Shortly after capture, the Chinese guards asked servicemen to jot down a few short pro-communist statements ("Communism is wonderful", and "Communism is the way of the future"). Many of the Americans were happy to oblige because the request seemed so trivial. A few weeks later the guards upped the ante and asked the prisoners to read the statements aloud to themselves. A couple of weeks later the Americans were asked to read the statements out to their fellow prisoners, and to engage in mock debates arguing why they believed the statements to be correct. Finally, fresh fruit or sweets were offered to any soldiers who were prepared to write pro-communist essays for the camp newsletter. Once again, many of the prisoners were happy to oblige.

The Chinese did not have to resort to arcane brainwashing techniques. Instead, they simply ensured that the prisoners were encouraged repeatedly to support communism, and then leave them to develop beliefs that were consistent with their behaviour.

Researchers have seen the same effect in their laboratories. In some studies participants have completed questionnaires about their political beliefs, and then been asked to give a short speech in favour of a political party that they oppose. Two weeks later the participants had come to believe that perhaps the opposition party wasn’t so bad after all.

The same procedure has been used in many different contexts, with people presenting talks about abortion, smoking, drink-driving, and greater police powers. On each occasion, behaving as if they believed a certain argument achieves what a hundred rational reasons couldn’t, quickly changing their attitudes in favour of the position they were asked to support. Indeed, the depth of change is such that the participants often deny ever holding their original opinions, and if they are shown their original questionnaires they argue that the forms have been faked or claim that they misread the questions.

The same effect can be used to influence entire populations. Saying "Heil Hitler" every day would have encouraged many ordinary Germans to become more open to Nazi ideology. Having people repeatedly sing a national anthem will make them more patriotic. Making children pray every morning will increase the likelihood of them adopting religious beliefs.

Each time people may feel as if they are simply "playing along". In reality, their behaviour is having a deep and lasting effect on their thoughts and beliefs.

Professor Richard Wiseman is a psychologist at the University of Hertfordshire. 'Rip It Up' is published by Macmillan on 5 July.