Most of us think we know the telltale signs of a liar—shifty eyes, sweating, a long, winding story that seems highly improbable. The stereotypes are even cross-cultural: a 2006 study  done at the Texas Christian University found that similar perceptions of liars exist in over sixty countries.
In reality, however, there is not one behavior all liars exhibit and some behaviors we associate with lying could mean something else entirely. Because of this, few people are very good at spotting liars. Even the so-called “experts”—body language professionals, customs officials, etc.—are only right about half the time.
Deciphering a liar from a truth-teller is not completely hopeless; it just isn’t as easy as is seems.
Can’t Smile While You’re Lying
After taking an online test  to see if I could discern a fake smile from a real one, I realized that, like most people, I’m not very good at it. I got about 50 percent correct—not much better than chance alone. Although it can be hard for the untrained eye to detect a genuine smile from a false one, they are different, and some small clues can help you pick out which is which.
Much of this has to do with paying attention to subconscious control of facial muscles. Paul Ekman, a professor of psychology at UCSF and author of Telling Lies, developed a way to distinguish real emotions from fake ones by identifying the role of specific muscles in our face. The Facial Action Coding System (FACS)  looks for things called “microexpressions,” which are brief displays of real internal feelings.
Part of what he found is that although fake smiles and genuine ones use some of the same muscles, real smiles use muscles generated by the unconscious brain, meaning there are certain facial actions we can’t fake.
For instance, in both fake and real smiles, the zygomaticus major muscle pulls the cheeks upward. However, with genuine smiles, the parts of our brain that process emotion also raise the orbicularis oculi and pars orbitalis muscles, which raise the cheeks and cause the eyes to crinkle.
Looking back at the test faces, I noticed that sometimes the deceitful smilers also seemed to have squinted or creased eyes. The key is to look at the “eye cover fold,” which is the skin between the eyelid and the eyebrow. With real smiles, this area moves downward and the eyebrows lower a bit as well. It’s hard—if not impossible—to consciously try to do this.
Body movements can also hint at a liar. In a 2003 study on cues to deception, UC Santa Barbara psychology professor Bella DePaulo found that liars are tenser and fidget more than those who are telling the truth.
According to an article in the Journal of Accounting by Joseph Wells, founder of the Association of Certified Fraud Examiners, this may be a result of our innate “fight or flight” system, which triggers movement in times of stress. He contends that some people may change positions when asked difficult questions or in the middle of a lie. Thompson also notes that sometimes liars cover their mouths when they are telling a lie.
DePaulo also found that liars are generally more unpleasant and complain more than non-liars do, although this is obviously a personality trait that some people have regardless of whether they’re lying or not.
Studies have shown that liars also change their speech patterns and word choice. Liars will sometimes use distancing language, meaning they use fewer first person pronouns and more third person. A 2003 study  by researchers at the University of Texas showed that in addition to using fewer self-references, liars also used more negative emotion words, such as “angry,” “frustrated,” or “fear.”
Liars might also need to stall in order to come up with details about their story. A 2001 study published in Neuroreport found that lying was associated with longer response times.
The pitch of one’s voice may also increase when he or she is lying, which could be a clue of storytelling.
One of the challenges in detecting lies through body language is that not all lies are created equal—some are white lies; others are fabricated under extreme duress where the stakes are high. Therefore, not all reactions are the same. One of the biggest challenges for lay people trying to detect them is that we don’t expect them—most of us think we’re getting the truth.
Updated August 28, 2009