You can keep your cards close to your chest, but your eyes will still give you away, according to new research. The study, published this week in the Psychonomic Bulletin and Review, reveals how people automatically shift their gaze to the left or the right depending on how good their hand is while playing blackjack.

When tracking the eye movements of 58 participants as they played a computerized version of the casino classic, the researchers discovered that those holding high-value hands momentarily looked to the right when calculating their odds of going bust, while those with low hands glanced to the left.

Importantly, these eye movements were not in response to the value of any single card, but occurred when the players were doing mental arithmetic in order to work out the combined value of all the cards in their hand.

Blackjack is a game in which players start off with two cards, and then have the option to either “stick” with those cards or “twist” and receive an extra card. The aim of the game is to end up with a hand that has a combined value that is as close to 21 as possible, although twisting too many times can be risky, as going over 21 means you are “bust” and therefore lose your bet.

Though the individual cards themselves can be either high or low, the horizontal eye movements seen in the experiment did not correlate to these values, but to the overall value of a player’s hand. For instance, if a player was dealt a 2 followed by a 10, they tended to glance to the left after receiving this second card, as their overall hand had a value of 12 – which is relatively low – despite the fact that the last card they were dealt was a high one.

The study authors therefore conclude that the eye movements had nothing to do with being dealt a particular card, but occurred as players added up their cards in their head.

Exactly how this information can be used by sneaky gamblers looking to perfect their poker face is not yet clear, as the researchers don’t know whether it is possible to override this seemingly involuntary action, or indeed if it can be detected by opponents.

"Whether our findings will help blackjack players in real life still has to be investigated," said study co-author Kevin Holmes. "The relatively small differences in absolute gaze position we found here may be undetectable to the naïve observer. Perhaps following training, observers could rely on gaze patterns to infer hand value."

^{Image: This player is probably looking to the right. okanakdeniz/Shutterstock}