What are you most afraid of? Sars? Terrorism? A plane crash? You're far more likely to die in a car accident than a plane crash, and yet many people fear the latter far more. Obesity, heart disease and strokes carry off many more of us than Sars is ever likely to. Yet few of us make major changes when our waistlines, blood pressure and cholesterol levels start creeping up. This is all about risk perception - which psychologists say is poorly understood and in need of better study. There's already a body of work on it, particularly on why we're not afraid of the things that are most likely to kill us. It's been called optimism bias - the idea that the normal human psychological response to risk is to accept that it's out there but to assume 'it won't happen to me'. But when it comes to less normal risks, such as terrorism or new infectious diseases, this optimism bias fails to kick in. Most of us develop an attitude called probability neglect. There were some outstanding examples during Sars: drivers without passengers, with all the windows up and a mask firmly clamped to their face; people who wore masks with their noses exposed or pushed down to their chins. The mask became like a talisman, rather than a simple means of reducing the risk of infectious transmission. We're not the only group to respond illogically or incorrectly to threats. In Israel, after a scud missile attack during the Gulf war, 825 out of 1,059 emergency admissions were caused by panic, overreaction, poor knowledge and the wrong use of protective equipment. Most injuries and deaths were a result of suffocation caused by wearing gas masks with airtight caps on. Many feared, with no proof, that the missiles were chemical weapons and injected themselves with atropine, poisoning themselves. Others were trampled in the panic to reach safety, while some collapsed with acute stress reactions - the result of fear of fear. We now know a little more about risk perception and risk communication - so much so that the latter is an important part of the role of the Centre for Health Protection, set up after Sars. Giving people the right information, clearly and early, seems to limit the danger of panic and irrational behaviour. But how do you know that the information you're getting is right? During the panic that followed the September 11 attacks, Americans were told to seal their homes with duct tape in the event of a nuclear or chemical attack. Other advice included staying in their homes - but at the same time to get as far away as possible from any source of radioactivity. Much of this stuff came over the internet - a lot faster than any clear information from the authorities. Next time someone tells you that the borders are being closed and that you'd better strip the supermarket shelves, check before your pass it on. And when your body is giving you important information about real threats - chest pain, high blood pressure, increasing body weight - take it seriously because these threats can be dealt with. If they're ignored, they can lead to the sorts of nightmares they don't make action movies about - but ones that are more likely to end your life.