5 Things People Do With Their Mouth That Mean They're Lying, Experts Say

1 | They purse their lips.

If someone's mouth appears tense, it's a major tip-off that they might be lying.

"The lips are full of nerves and are highly vascular, and as the brain's emotional control center, the limbic system, controls them, they are challenging to manage during periods of heightened emotional reactions, such as stress or attempts at deception," explains Ellie Borden, registered psychotherapist and clinical director of Mind By Design.

"This is an indicator to pay attention to when observing the response of a person who has no reason to feel stressed or anxious when answering a question," Borden adds. The person might assume this position after hearing your question and before responding or immediately after telling you a lie.

2 | They bite their lips.

Another common stress indicator is when someone bites their lip, according to Joe Navarro, an ex-FBI agent who served on the National Security Division's Behavioral Analysis Program.

In a Psychology Today article, Navarro says that lip biting "is one of the ways that we pacify ourselves when we are stressed. It helps to relieve tension that may be minor and transitory." You might also observe someone biting the inside of their cheek if they're tense and lying.

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3 | They fold their lips.

This mouth movement might appear more dramatic than pursed lips.

"When someone quickly folds in their lips before they speak, it signals that they may be holding back information or are about to state something that may not be fully true," says Karen Donaldson, celebrity communication, body language, and confidence coach. "It signals that they are not saying what they really want to say."

Navarro refers to this as "lip compression" and says he very often saw that when a person was distressed, "their lips begin to disappear, becoming very thin as vaso constriction takes place. Under extreme stress they disappear completely or are compressed together."

4 | They smile from ear to ear.

A 2018 study from the University of Rochester found that there were five kinds of smile-related faces people tend to make when answering a question. The smile that was most associated with lying was the "Duchenne" smile, or a large smile that involves cheek, eye, and mouth movements.

The researchers noted that the Duchenne smile is involuntary. So, if the person you're chatting with seems unusually excited, they might be trying to deceive you. People telling the truth were more likely to contract their eyes and were less likely to grin with their mouths.

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5 | They speak with a high-pitched voice.

When people are nervous, the muscles in their vocal cords tend to tighten, which can lead to a higher-pitched voice. A 2012 study published in the journal Psychiatry, Psychology, and Law found that participants' pitch increased when they lied.

"These findings emphasize the utility of pitch as a marker of deception because it may be less susceptible to behavioral control than physical markers such as gaze behavior," the study authors wrote. In other words, this involuntary response is a pretty major indicator that someone is telling a fib—so listen up.

There are also a few key phrases you'll always want to note when determining if someone is being truthful. According to Donaldson, a liar might use generalized or inflated statements, such as "I always" or "I've done that a million times" and leave out details.

They might also overstate their sincerity. "They say things like, 'Let me be clear,' 'to be perfectly candid, that is not what I said,' and 'to be honest,'" says relationship expertSameera Sullivan. "These statements are intended to sound credible, so the liar can trick you." Keep your eyes and ears peeled so you can always catch them in their tracks.