A change in speech patterns
One telltale sign someone may not be telling the whole truth is irregular speech. According to Gregg McCrary, a retired FBI criminal profiler, a person’s voice or mannerisms of speaking may change when they tell a lie, as reported in Forbes.
McCrary first identifies a person’s regular speech patterns and mannerisms by asking typical, straightforward questions, such as what their name is or where they live. This allows him to see any changes in speaking or characteristics when he asks more challenging, interrogative questions.
Not saying enough
When truth-telling witnesses describe what they saw and are asked: “Is there anything else?” more details are revealed. But when liars are asked to go beyond their prepared stories, few other details are offered.
Researchers quoted in the American Psychological Association (APA) refer to these people as “liars who deceive by omission,” who, when asked to answer questions or provide more details, typically offer less than those telling the truth. This can be quantified through transcripts of phone calls, witness statements, or noticed by an absence of descriptive words in a conversation.
Another way researchers verify the truth is by asking people to tell events backward. Truth-tellers will stick to the same story while offering more details, while liars often get tripped up and create a different story without adding detail to the original.
Saying too much
On the flip side, researchers from Harvard Business School determined that liars trying to deceive stretch the truth with too many words. Since such a liar may make up things as they go, they may also add excessive detail to convince themselves or others of what they are saying. They may also embellish with words that a person telling the truth wouldn’t think of adding.
Other linguistic cues revealed in this study show that liars use more profanity and third-person pronouns (e.g., he, she, and they) to distance themselves from any first-person (e.g., I, my, mine) involvement.
Direction of their eyes
Much has been discussed on the topic of truthfulness and eye contact. A commonly-held cultural belief in the United States is if a person isn’t making eye contact, they aren’t telling the truth, whereas, in other cultures, eye contact can be considered untrustworthy in a given context.
Covering their mouth or eyes
Many people want to cover up a lie or hide from their reaction to it, which may be why they put their hands over their eyes or mouths when letting an untruth out. According to former CIA officers in their book Spy the Lie, others may even completely close their eyes when telling a lie, as reported in Parade Magazine.
This could be especially true when it’s in response to a question that does not require a lot of reflection.