It's probably how your father or grandfather knew if someone was telling the truth: the ol' look 'em in the eyes test.
Now new research backs up the notion that it's harder for someone to make eye contact while fibbing.
Psychologists at the University of Tampere conducted an experiment in which participants had to play a lying game on a computer against an opponent. The game briefly presented participants with a view of their opponent's face before they made a move. In some game trials the opponent looked the participant in the eyes and it was found that lying in the game was reduced by this direct eye contact compared to other trials in which the opponent did not make eye contact.
"This was the first study to demonstrate the effect by using actual eye contact with another person and by measuring not just any form of dishonesty, but lying," says Jonne Hietanen, the first author of a report outlining the findings of the experiment in the journal Consciousness and Cognition.
Other studies have used images of eyes and found that even static images reduced people stealing drinks or bicycles, among other similar experiments.
Still, even the most well designed experiment may not be able to replicate the conditions found in real life.
"Because the results were obtained in an experimental situation, one must be careful not to draw too far-reaching conclusions," Hietanen said.
Still, it's good to know that grandpa's instincts were correct and there is value to looking someone in the eye before you take them at their word.