How to Tell If Someone Is Lying—Is Body Language the Key?

This article is an excerpt from the Shortform book guide to "Talking to Strangers" by Malcolm Gladwell. Shortform has the world's best summaries and analyses of books you should be reading.

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Do you know how to tell if someone is lying through their body language? Is body language really the key to detecting a lie?

We’ll cover why it’s hard to learn how to spot a liar through body language and what signals to watch for (and why mismatching might throw you off).

How to Tell If Someone Is Lying, Using Body Language

Meeting a stranger can make it more challenging to make sense of that person than not meeting him. That’s because people assume transparency in others. It’s hard to learn how to tell if someone is lying using body language because sometimes, the body language is not what we’d expect.

Transparency refers to the assumption that the way people present themselves outwardly (through behavior and demeanor) is an accurate and reliable representation of their inner feelings and intentions. But that’s an unrealistic assumption to make when dealing with strangers.

Before we explore why we can’t assume others will be transparent, let’s look at how transparency works.

Transparency and How It Works

Facial expressions are one of the primary ways that we interpret a stranger’s feelings (because we mistakenly assume that a person’s demeanor is an accurate representation of his feelings). This mistake is one reason it’s hard to know how to tell if someone is lying through body language.

Coded Facial Actions

Psychologist Jennifer Fugate is an expert in the system of coding facial actions (referred to here as FACS). FACS assigns a name, or “action unit,” to each of the 43 possible muscle movements of the face. This action unit is used to notate and score people’s facial expressions like music is scored by notes on a page. For example: 

  • The kind of soft, insincere-seeming smile you would get from a flight attendant has a score of AU 12, based on the facial motions used to create that expression: Pulling up the corners of the lips, and leaving the rest of the face still. 
  • A genuine-seeming smile has a score of AU 12 plus AU 6. The corners of the lips pull up and the cheeks raise to crinkle the eyes. 

It is important to note that these smiles only seem to be insincere or genuine based on how they look. People’s expressions are not always transparent, as discussed below. It’s hard to know how to spot a liar through body language.

 Transparency and Friends

Friends is one of the most successful and recognizable shows of all time—in part because the characters are completely transparent. To prove the connection between transparency and the success of Friends, Jennifer Fugate watched the first scene of season five, episode 15 and performed a FACS analysis of each character’s expressions throughout. 

At the beginning of the episode, Ross sees Chandler and Monica in a romantic moment. This is significant because Chandler is Ross’s best friend and Monica is Ross’s sister. Ross rushes to Monica’s apartment to bust in and stop them. He is frantic. Joey and Rachel come into the scene, while Chandler hides behind Monica to stay out of Ross’s warpath. Monica and Chandler announce that they are in love. Slowly, Ross comes around to being happy for them.

That might seem like a lot to keep up with, but Friends is incredibly easy to follow. Why is that? Because the characters are transparent, as Jennifer Fugate proved with her FACS reading of the scene. 

The Results
  • In the moment that Ross sees Chandler and Monica embrace, his face reads: upper-lip raise, lower-lip depress, lips part, and jaw drop—all at the highest intensity rating. This is the stereotypical look of anger and disgust.
  • In the moment that Monica tells Ross she’s sorry for surprising him but that she’s in love with Chandler, her face reads: 1 + 2, which communicates classic sadness, then eyebrow raise, then the AU 12 flight attendant smile. This combines to express that she is sad to upset Ross, but happy to be with Chandler.   
  • In that same moment, Ross’s face reads: 1 + 2, classic sadness, then 1 + 12, the beginnings of happiness. His sadness has shifted into joy when he sees that Monica is happy. 

Fugate’s FACS reading proves that the actors in Friends portray every emotion their character goes through inwardly directly through their facial expressions—the characters are completely transparent. The show’s popularity is evidence that people like dealing with transparent people (and characters). Transparency makes people and their stories easier to understand. 

When we meet strangers, we tend to believe that they’ll be as transparent as the characters on Friends. This is the “Friends Fallacy.” It’s a fallacy because real life isn’t like an episode of Friends. In reality, strangers often aren’t transparent. So why do we assume they are? Can we learn how to tell if someone is lying through body language?

Is Assuming Transparency Beneficial? 

In The Expression of the Emotions in Man and Animals by Charles Darwin, Darwin argues that it is beneficial to human survival that people are able to quickly and accurately communicate emotions to one another. 

The ability to smile, frown, and wrinkle the nose in disgust are some examples of how the human face evolved as a tool to represent internal feelings. This will probably strike you as a relatively obvious principle. After all, children everywhere naturally smile when they’re happy and frown when they’re sad, and that helps them get what they need to survive. So it seems reasonable to assume transparency. It seems like we should be able to learn how to spot a liar using body language and facial expressions.

But you should be careful not to assume that every stranger you come across will be transparent. That assumption requires everyone you meet to express themselves in the same predictable ways. Unfortunately, that is not the case. You can’t always be sure you know how to tell if someone is lying through their body language.

How to Tell If Someone Is Lying—Is Body Language the Key?

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Like what you just read? Read the rest of the world's best book summary and analysis of Malcolm Gladwell's "Talking to Strangers" at Shortform .

Here's what you'll find in our full Talking to Strangers summary :

  • Why we don't understand strangers
  • How to talk to strangers in a cautious way so you don't get fooled
  • How Hitler deceived so many world leaders

Amanda Penn

Amanda Penn is a writer and reading specialist. She’s published dozens of articles and book reviews spanning a wide range of topics, including health, relationships, psychology, science, and much more. Amanda was a Fulbright Scholar and has taught in schools in the US and South Africa. Amanda received her Master's Degree in Education from the University of Pennsylvania.

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