How can a helmet make cycling more dangerous? It seems obvious that a well-designed, foam-lined hard hat will soften the blow of some impacts. But hang on – a clever psychological experiment reveals you may actually be increasing your overall level of danger.

Psychologists at the University of Bath have now shown that people take more risks when they wear a helmet. Studies of injury data suggested this before but, for the first time, it’s been demonstrated in the lab.

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The experiments reveal that when you clip those straps together under your chin you make yourself feel safer. So, without realising it, you compensate by taking more chances on your bike and are rewarded by getting more of a thrill.

The bottom line is that you may be protecting your head but you are riding in a way that exposes your whole body to more danger and chance of injury.

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It’s a fascinating result that is bound to stir up the heated argument about helmets. In some parts of the world the law says cyclists must wear helmets. It’s like this in some Australian and Canadian states – and even Jersey in the Channel Islands mandates helmets for children.

This new study means there will be even more questions about those laws. What is the point of protecting someone’s head if it makes them ride more dangerously?

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Testing the theory of risk compensation among helmet wearers is a challenge. The volunteers cannot be allowed to know what is being tested in case it influences their behaviour. So Dr Tim Gamble and Dr Ian Walker of the University of Bath devised a fiendish way to stop them realising.

They lied to 80 human lab rats by telling them they were joining eye-tracking experiment which required them to wear a head-mounted camera. The camera was attached either to a helmet or a baseball cap and the choice of head-gear was made randomly.

Risk-taking was measured by allowing the volunteers to choose when to stop a balloon from being inflated. Those who wore helmets allowed it to be blown up more. The psychologists reason that, even though they were not in a dangerous situation, people without protective helmets were more cautious – taking fewer risks and not seeking as much excitement.

Bicycle helmet wearing can increase risk taking and sensation seeking in adults, by Tim Gamble and Ian Walker, to be published shortly in Psychological Science.

Max Glaskin is an award-winning freelance journalist who tweets about cycling and science as @CyclingScience1 and is the author of Cycling Science (published by Frances Lincoln UK, Chicago University Press USA, and seven other languages).