Now you can recognise if a person is lying about recognising someone they know through their eye movements, according to a new study.
The finding could be valuable to police when trying to confirm key identities in criminal networks such as terrorist cells or gangs, researchers said.
Using eye tracking technology, researchers from the University of Portsmouth in the UK found that people's eyes move in a different pattern when looking at faces they recognise.
"Criminal accomplices often deny that they know other members in their networks. However, if a co-conspirator denies recognition in this way, their eye movements when viewing photos of those suspects, may reveal this type of lie", said Ailsa Millen, lead author of the study.
The researchers recorded the eye movements of 59 participants while looking at 200 digital colour photographs of familiar and unfamiliar faces.
Familiar faces included people the participants knew in real life, famous celebrities and those only seen briefly before the experimental trials.
Sometimes the participants lied about whether they recognised the photos, sometimes they told the truth.
"We found that people's eye movements were different when looking at photographs of faces they knew well compared to those they did not know, despite verbal reports denying recognition.
"When a participant looked at a face they recognised their eyes moved in a different pattern with fewer fixations.There is substantial evidence to suggest that this pattern is involuntary, which means it could be hard to control or fake", Ailsa said.
Ailsa and colleagues modified a memory detection technique known as the Concealed Information Test (CIT) to conduct the research.
The CIT differs from traditional lie detection techniques that attempt to directly assess guilt based on arousal.
It is considered the gold standard of laboratory methods to detect concealed recognition and has been researched and validated over decades of scientific studies.