The way a person’s eyes move can show whether or not the person recognizes someone else -- even if they try to hide it. That is what a team of international researchers reported recently.

The finding could be important for criminal investigators, who often seek information from uncooperative witnesses or suspects.

Law enforcement officers often turn to lie-detector tests during criminal investigations. But the test results are not always correct.

When asked questions critical to a criminal case, a fast heartbeat or sweaty, sticky hands might make a nervous truth-teller appear to be lying. And, an experienced liar might be able to control those signs and avoid suspicion.

Our eyes look at a familiar face differently than they do an unfamiliar one. When people look at unfamiliar faces, their eyes usually move from one feature -- like the shape of the nose -- to other features. Their eyes stop quickly and often as they try to identify the unknown person.

When looking at a familiar face, people usually direct their attention to just a few facial features.

The researchers wanted to know if people could control their eye movements when attempting to hide the fact that they recognized a face.

Alisa Millen is a psychology researcher at the University of Stirling in Britain. She led the team that carried out the study.

Millen said, "Humans are experts at familiar face recognition. Recognition of a familiar face is fast and reflexive."

The researchers showed 48 students pictures of both strangers and professors they knew. They asked all of the students to try to appear honest while lying about recognizing familiar faces.

Half of the test subjects were told about a method that might help them hide their recognition. The researchers told those students to stop in the same areas when looking at both familiar and unfamiliar faces.

Their eyes would start on the person’s forehead. Next, they looked at each eye, then moved from one ear to the other, then down to the nose, mouth and chin.

In most cases, the students who were given suggestions for how to hide their recognition were not able to do so. When looking at a familiar face, their eyes still stopped for longer periods of time and had fewer stops in the inner parts of the face, the researchers noted.

Millen said that hiding “markers for facial recognition in eye movements is difficult, especially if you know that person well." She added the more the students tried to hide recognition of a face, “the more apparent it was.”

Millen hopes that her findings can someday be used by law enforcement. She believes such knowledge could help investigators have a clearer understanding of who is connected to whom in a criminal case.

Deborah Hannula is a psychology professor at the University of Wisconsin-Milwaukee. She was not involved in the study. She said of the research, "It's incredibly important to detect whether someone knows something and isn't willing to reveal that in cases that have high importance, like terrorist investigations."

Hannula and Millen both agree that while the method is promising, it needs a stronger experimental base before it can be used by law enforcement.

Millen noted that she and her team explored only one method of trying to hide recognition. Many other methods could exist, she said. Some might even be more effective.

It is also not yet clear what effect the degree of familiarity has. In order for the method to be used in criminal cases, it needs to be effective for faces that are very familiar as well as only partly familiar.

The research appears in the scientific publication Cognitive Research: Principles and Implications.