The science of why we lie "As any good poker player will tell you, it's possible to control your face," says Dr Seager. But you might start jiggling your leg without even realising, or one of your fingers might start twitching. Idiosyncrasies differ from person to person. If someone's deviating from normal behaviour there must be a reason - perhaps they're lying, or they may just be uncomfortable. Although there are some very accurate lie detectors, humans are not particularly good at judging whether someone else is lying. Whether you believe you're good at spotting falsehoods or not, most people can tell when another's lying around 54 per cent of the time. While most of us tell white lies, we are more willing to tell more serious untruths - such as misleading our boss - than we were 20 years ago. We also lie to protect ourselves and others, and to inflate our ego. "Some people lie because they get a kick out of pulling the wool over other people's eyes - it gives them a feeling of power," says Dr Seager. But lying can go beyond everyday normality into the symptom of a pathological condition. Psychiatrist Dr Cosmo Hallstrom says that incessant lying can be the sign of a deep-seated personality disorder, such as psychopathy, sociopathy or borderline personality disorder. "Some people live in a fantasy world and don't tell the truth - people who for various disturbed functions in their psychological make up feel the need to live a false existence," says Dr Hallstrom. Psychopaths, for example, have no remorse or conscience. They focus on the short-term gain, they have no guilt and they live for the moment impulsively and without thinking of the consequences. In one form of pathological lying, known as Munchausen syndrome, people try to persuade doctors that they have a serious medical condition. "All doctors encounter such people - they become the centre of attention for a while because they have a condition no one can understand," says Dr Hallstrom. They go into hospital and produce a HIV positive test, or mix some blood with their urine and persuade the doctors they have bleeding in the bladder - and the lies get more and more fantastic. While some pathological liars lie incessantly to exaggerate their own importance, others may struggle to accept reality and the difficult truths of their lives. Either way, Dr Hallstrom says the underlying condition is usually deeply ingrained, and would take serious work from the individual to treat or overcome. "A suitable therapist may help you but it's difficult to deal with because the first thing you start doing is lying to your therapist," he says. It's hard to know how many pathological liars are among your friends or colleagues, as most are extremely convincing liars. And as incessant lying is symptomatic of a clinical condition, pathological liars may not behave the same way as most people who lie on occasion - such as showing relief when the conversation topic is moved away from their lie. We don't know how many people are pathological liars, as few will admit to the condition. But if there are some people who lie constantly and the average person lies ten times per week, logic suggests that there are highly moral individuals who hardly lie at all. Most mere mortals are tempted to dip into falsehood now and again to avoid a difficult situation or make a friend happy. But be warned: most people try to convince themselves into believing their own lies. So if you start losing track of those growing untruths, you may distort your entire perception of reality.