A POKER face. It is the expressionless gaze that gives nothing away. To win at poker, the face must be mastered, and master it is what the best players try their best to do. But a study just published in Psychological Science by Michael Slepian of Stanford University and his colleagues suggests that even people with the best poker faces give the game away. They do so, however, not with their heads but with their hands.

Mr Slepian made his discovery when he showed 78 undergraduate volunteers video clips of players placing bets at the 2009 World Series of Poker. (Bets in poker are placed by pushing chips into the middle of the table.) The clips were 1.6 seconds long, on average, and featured different parts of the players’ anatomies. Some showed everything visible from the table up: chest, arms and head. Some showed just the face. And some showed only the arms and hands. Each volunteer watched only one of the three types of video, but was shown several examples.

After each viewing, volunteers were asked to rate the quality of the player’s hand on a seven-point scale. Then, when they had finished watching all the clips, they were asked to rate their own experience with poker on a similar scale.

Mr Slepian found that students were poor at judging the quality of a player’s hand when shown just that player’s face. Indeed, he noticed a negative correlation of 0.07. This is not huge (a perfect correlation is 1.0). But it meant there was a statistically significant tendency that the better a volunteer believed the hand to be, the worse it actually was. When a player’s whole posture was considered, this misapprehension went away: if a volunteer could see everything about a player from the table up there was no correlation between his judgments of a hand’s value and its actual value. When a volunteer could see only arms and hands, however, Mr Slepian found a positive correlation, of 0.07, between his guesses and reality.

To confirm his discovery, Mr Slepian re-ran the experiment with a different set of clips. The results were the same. Students, even those who were poker novices, could judge the quality of a professional poker player’s cards from the behaviour of his hands. The next question was, how?

Mr Slepian knew from previous studies by other people that anxiety has a tendency to disrupt smooth body movements, and he suspected this might be the explanation. To find out, he showed 40 new volunteers the clips he had used in the previous experiment. Rather than asking them to judge the quality of a player’s cards, however, he asked them to rate either that player’s confidence or how smoothly the player pushed his chips into the middle of the table.

He found that when students rated players as being confident or having hands that moved smoothly, the cards they held were likely to be good. There was a positive correlation of 0.15 when the students considered confidence and of 0.29 when they looked for smooth movement, so they were actually more capable of determining hand quality from these variables than when asked to estimate it directly. The moral of the story for players, then, is don’t look your opponent squarely in the eye if you want to know how good his cards are. The secret of his hand is in his hands.