As he replays the recording in slow motion, he points out facial expressions indicating that Clinton, if not lying, is certainly feeling uncomfortable.

Asked if he kissed the intern's breasts, his nose wrinkles in the bat of an eye. That's a sign of disgust, Freshman says. Asked if he used a cigar on her as a sexual aid, his eyes pop wide open in surprise so rapidly it's easy to miss. Clinton replies repeatedly that he stands by his statement: he did not have "sexual relations" with Lewinsky. (He later admitted that he did; it had been a matter of definition.)

Looks of disgust and surprise are significant, according to Freshman, because lie detection is complex, and we often mistake the signs. "We're terrible at spotting lies," he says, adding that studies show an average success rate of between 52 and 55 per cent.

Some myths about lying are commonly held to be true. We may think a person is deceitful if they fidget. "When I trained federal judges who decide disability cases and labour disputes, they would write opinions on why they didn't believe someone. They called [fidgeting] 'sit-and-squirm evidence'," he says. "But the opposite is true in all research. When people are lying, they are much more rigid" - like Clinton in his video testimony.

Another misconception is that a person is lying if they look left or right, rather than make direct eye contact. They may be simply wondering what to wear for dinner.

The real secrets to detecting a lie, Freshman says, include spotting "micro-facial expressions" that communicate key emotions. "There are millions of things that go on in the face, but not all are related to emotions."

Some people pout their lips to express doubt, for example. Or we may raise our eyebrows to show we are listening.

Micro-facial expressions - which can pass in the blink of an eye or last several seconds - are common to all races and cultures, and are even seen in primates, he says. In general, they can represent seven emotions: disgust, anger, contempt, fear, surprise, happiness and sadness/distress.

Freshman illustrates them with photos. The knit brow of anger and disgust are similar, he notes, so the nose wrinkling of disgust is key to differentiation. Fear and surprise are alike in the wide eyes, though the mouth is more open and the eyebrows come down in fear. "We often fear surprises," he says.

Whether in a courtroom, police station, arbitration session, business meeting or any number of situations, he says, recognising these emotions unlocks clues to a person's true motives - whether they're being honest, lying, or simply not telling the whole truth. Job interviewers would find the technique useful in screening candidates. Someone seeking a job with a bank, who displays signs of anger - "and there are a lot of angry people in banking"- shouldn't be considered for front office or account managing work, he says.

Another video in Freshman's arsenal is an interview with a university dean. "He says he's worked here, here, and Northwestern. When he says 'Northwestern' he crinkles his nose. What happened at Northwestern? If I were thinking about hiring that person, I'd want to look into that."

But lie detection is more complicated than just catching micro-facial expressions. A person's "baseline", or normal, behaviour has to be considered. Some people have naturally sad faces; others always look angry. In most photos of former US vice-president Dick Cheney, for example, he has the one-sided smile of contempt on his face. "Cheney just looked contemptuous all the time," Freshman says. "So changes in baseline is a clue [to identifying lies]."

Consider a person's normal physical movement; the speed of their speech. "I tend to meander. So if you ask me a question and I directly answer it, it's more likely that I'm hiding something," he adds.

Baseline behaviour was cited by a lawyer for Kato Kaelin, American football star OJ Simpson's sole alibi in his double-murder trial. Kaelin preceded his answers with a lengthy pause, indicating he might have been fumbling for an answer. But the defence said that was normal for him. "He isn't the brightest light bulb in the chandelier," the lawyer said.

Freshman counts a number of big businesses as clients. When a private equity firm asked him to help set up its entire office system, one outcome was to furnish all its conference rooms with glass-topped tables. A lot of people can control their upper body but move their legs unconsciously at crucial questions in meetings, he says.

"You might ask: 'Which other companies are looking to acquire you?' They may name three companies, and they get to the fourth and their legs start shaking. So they don't have four offers; they have three."

In coaching staff from the US Department of Homeland Security who were working at airports, he says simply looking for people who showed signs of anxiety was not going to help.

"You don't want to make people uncomfortable. With the kind of environment where you're harassing people, or just being inconsiderate when they're afraid of flying, for example, you are going to think they are lying when there are so many other reasons they look that way."

Perhaps unsurprisingly, Freshman says he is often approached at the end of a seminar for personal advice. Some want to find out if their wives are cheating on them; others ask for dating advice.

"I get that a lot. I teach them that it's important to know if someone is lying, but it's also important to know if this person is compatible with you or not. The first thing is, see if she often shows you contempt" - that one-sided smile.

A relationship in which one person shows the other contempt too often is usually over within a year, he claims.

Freshman's interest in deception was born of personal tragedy, following the death of his mother, who suffered from depression: "I did this research to figure out what people are really feeling. Are they actually at risk?"

People who are depressed may tell others they are fine when they are actually suicidal, he points out. There are many reasons why people lie; sometimes it's because the truth isn't as flattering or acceptable.

Our inability to detect deceit may stem largely from bias, in terms of who we do and don't trust.

"There are some people, no matter what they do, who are likely to be believed," Freshman says. "Good-looking people are more likely to be believed. That's partly genetic; their faces may be symmetrical. People who are like you are more likely to be believed. But then people are more accurate at detecting lies among people who are like themselves. So Asians are better at detecting lies among Asians than Caucasians, and vice versa."

People with anxious faces, however, are less likely to be trusted.

Freshman says he is ultimately motivated by trying to help reduce depression in society, promote happiness and improve relationships.

He mentions the recent unrest in Ferguson, Missouri, following the shooting by a policeman of Michael Brown, and reignited when a grand jury failed to indict the officer.

"There's so much unconscious, unthinking bias. What are the signs that this person is actually dangerous, versus, 'Oh, right, here's this African-American person, let me shoot him six times'. Imagine if that person wasn't showing any signs of anger in his face. Instead he was showing signs of shame or sadness."

On a personal level, improving human relationships could be something as mundane as asking a friend where they want to go for dinner.

"You ask, 'How about Malaysian?' and they say, 'Oh, Malaysian's fine', but they shrug slightly with one shoulder. They're not really lying; they're just not thrilled with Malaysian. So you say, 'What about Greek?' and they light up with a smile that involves their eyes. Suddenly you're both happier."