The Tasmanian tiger, or thylocine, was a carnivorous marsupial (a meat-eating mammal which carries its young in a pouch). It was given the name 'tiger' because it had striped fur, and because it was ferocious. Between 24 million and 15 million years ago, many types of thylocine roamed across Australia, their powerful jaws playing a role in maintain a balance in the ecosystems of their day. Some species were fox-sized, while others were barely the size of kittens.

But when a period of climate change cooled Australia about 12 million years ago, the numbers of these ancient thylocines began to decline. By about 3 million years ago, only one species was left. About 4,000 years ago, these vanished completely from Australia mainland, so that Tasmania, a large island to the south of Australia, was then the last remaining place where thylocines existed. They ruled the animal life of that island unchallenged until Europeans with sheep, dogs and a great indifference to native flora and fauna, seem to have brought about their extinction. In 1936, the last captive Tasmanian tiger died in Hobart Zoo. Since then, many expeditions have searched for tigers in the Tasmanian bush, but no definitive evidence has been found. Despite this, there are many who keep searching.

In 1981, Dutch-born zoologist Hans Naarding was in Tasmania conducting a survey of Latham's snipe, a species of endangered bird. One night he saw an animal in the light from the searchlight mounted on his vehicle. He described it as about the size of a large dog, but with slightly sloping hindquarters and a fairly thick tail continuing straight on from its backbone. He said that it had 12 distinct stripes on its back, running down to the point where the tail began. He reported the sighting to Director of Tasmania's National Parks. When the news broke, said Naarding, I was besieged by televisions crews, including four or five from Japan, and others from the United Kingdom, Germany, New Zealand and South America.

Government and private search parties combed the region, but no further sightings were made. The tiger, as always, had escaped to its lair - a place that many insist exists only in the imagination. Others disagree, there have been more than 4,000 claimed sightings of the animal since it supposedly died out, and the average number of claims reported to the authorities each year is now 150. So is it out there? Even experts differ in opinion.

Randolph Rose, Associate professor of Zoology at the University of Tasmania, says that he dreamed of seeing a thylocine, but is now convinced that his dream will go unfulfilled. The consensus among conservationists is that any animal with a population base of less than 1,000 is headed for extinction within 60 years.

'Sixty years ago,' he says, 'there was only thylocine that we know of, and that was in Hobart Zoo. Take it from me, the tiger is gone.' But Dr David Pemberton, curator of zoology at the Tasmanian Museum, states that, despite scientific thinking that a relatively large number of animals is required to sustain a population, 'the Florida panther is down to a dozen or so animals, and, while it does have some inbreeding problems, is still ticking along!' After all, animals can be notoriously elusive. The strange fish known as the coelacanth, with its 'proto legs', was thought to have died out with dinosaurs 700 million years ago until a specimen was dragged to the surface in shark net off the coast of South Africa in 1938.

Wildlife biologist Nick Mooney has unenviable task of investing all so-called sightings of the tiger. It was Mooney who was first consulted in late February 2005 about authenticity of new digital photographic images of a thylocine allegedly taken by a tourist. On face value, Mooney says, this particular account of a sighting and the photographs submitted as proof amount to one of the most convincing cases for the species' survival that he has seen. Many other 'sightings' have been hoaxes, and many sincere seekers are victims of obsession. 'It is a blind optimism that something is, rather than cynicism that something isn't,' Mooney says. 'If something crosses the road, it's not a case of "I wonder what that was?" Rather, it is a case of "That's a thylocine!"'

However, Mooney treats all sightings on face value. 'I never try to embarrass people,' he says, '.. but the fact that I don't pack the car immediately after they telephone can often be taken as ridicule. Obsessive characters get angry that someone in my position is not out there when they think they thylocine is there.'

Hans Naarding, whose sighting of striped animal two decades ago was the highlight of a lifetime of animal spotting, remains puzzled by the time and money people waste on tiger searches. He says resources would be better applied to saving another endangered animal, the Tasmanian devil, and helping declining migratory bird populations. Could the thylocine still be out there? 'Sure,' Naarding says. I know the vast south-west wilderness of Tasmania well. They could survive ... [But] if this is the case, it will not be long before they do disappear completely. Naarding believes that any discovery of surviving thylocines would be 'rather pointless'. 'How do you bring a species back from extinction?' he asks. 'What could you do with it? If there are thylocines out there, they are better off right where they are.'

Forty years ago, the research psychologist Dr Paul Ekman was addressing a group of young psychiatrists in training when he was asked a question, the answer to which has kept him busy ever since. Suppose the group wanted to know, a particular patient swears they are telling the truth. They look and sound sincere. So here is the question: is there any way you can be sure they are telling the truth? Ekman did not know the answer then, but wanted to find out.

As part of his research, he had already filmed a series of 12-minute interviews with psychiatric patients. In a subsequent conversation, one of the patients told him that she had lied to him. So, Ekman sat and looked at the film but saw nothing noteworthy. Then he slowed it down and looked again. Then he slowed it even further. And suddenly, there, across just two frames of the film, he saw it: an intense expression of extreme anguish. It lasted less than a 15th of a second, but once he had spotted the first expression, he soon found three more examples in that same interview. He termed his discovery "micro-expression: very rapid intense demonstrations of emotion that the subject intended to be concealed".

Over the course of the next four decades, Ekman successfully demonstrated a proposition first suggested by Charles Darwin: that the ways in which we express rage, disgust, contempt, fear, surprise, happiness and sadness are universal. The facial muscles triggered by those seven basic emotions are, he has shown, essentially standard, regardless of language and culture, from the US to Japan and Brazil to Papua New Guinea. What is more, expressions of emotion are impossible to suppress and, particularly when we are lying, micro-expressions of powerfully felt emotions will inevitably flit across our face before we get the chance to stop them.

Fortunately for liars, most people will fail to spot these fleeting signals of inner torment. Of the 15,000 Ekman has tested, only 50 people, whom he calls "naturals", have been able to do it. But given a little more training, Ekman says, almost anyone can develop the skill. He should know, since these tests were completed in the mid-1980s and the first publication of his research, he has been called in by the FBI and CIA (among countless more law-enforcement and other agencies around the world), not just to solve cases, but to teach them how to use his technique for themselves. He has held workshops for defense and prosecution lawyers, health professionals, even jealous spouses, all of them wanting to know exactly when someone is not being 100 percent candid.

Most recently, Ekman's research has resulted in a new television series about the exploits of the fictional Dr Cal Lightman, a scientist who studies involuntary body language to discover not only if you are lying. But why might you have been motivated to do so? According to the publicity blurb, Lightman is a human lie detector, even more accurate than a polygraph test. Ekman concedes he was skeptical when the producer first approached him with the idea of turning his life's work into a TV series, and initially would have stopped the project if he could. In particular, he was fearful that the show would exaggerate the effectiveness of his techniques and create the quite inaccurate impression among audiences that criminals could no longer hope to get away with lying. In the worst-case scenario, he was concerned about unfair convictions, that one day someone not properly trained in his techniques might be sitting on a jury and wrongly find someone guilty of a crime simply on the basis of a television programme.

Ekman, incidentally, professes to have been a terrible liar ever since he was a small boy and observes that the ability to detect a lie and the ability to lie successfully are completely unrelated. He has been asked by people running for high office if he could teach them to become more credible with the public but has always refused to use his skills in that way on ethical grounds. He also insists that there are various kinds of lies. A "true" lie can be identified by having two essential characteristics: there must be a deliberate intent to mislead and there must be no notification that this is what is occurring. This means that an actor or a poker player isn't a true liar. They are supposed to deceive you, it's part of the game, and the same is true of flattery. He prefers to focus on the kinds of lies where the liar would be in grave trouble if they were found out, and where the target would feel properly aggrieved if they knew.