This is the lesson: What one thinks he is communicating hinges on a facial expression. Accordingly, a poker face is the expressionless gaze that gives nothing away. To win at poker, this idea counsels, one must master controlling an expressionless face.

In world poker tournaments the best players do their utmost to follow this adage. But a recent study published in Psychological Science by Michael Slepian and colleagues at Stanford University suggests that even those with the most stone–faced poker faces still give the game away. They do so not by their facial expressions, but with their hands.

Professor Slepian showed his students short video clips. Some depicted players from the table up; others just the face; yet others only the arms and hands. What he found was that students poorly judged a player’s hand when shown only the face. Statistically, the better a student judged the hand to be, the worse it actually was.

When a player’s whole posture was visible the misjudgment went away: Seeing everything about a player from the table up led to no correlation between a judgment of a hand and its actual value. When a student could see only arms and hands, however, Professor Slepian found a positive correlation of 0.07 between guesses and reality. This is extremely weak.

Even students who were poker novices could judge the quality of a professional player’s cards from the behavior of his hands. The question was, how? From previous studies Professor Slepian knew that anxiety tends to disrupt smooth body movements, and he suspected this might be the explanation. He showed a fresh set of volunteers the clips he had used in the previous experiment. Rather than asking to judge the quality of a player’s cards, however, he asked them to rate either the 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 was likely to be good. A positive correlation of 0.15 held when students considered confidence, and 0.29 when they looked for smooth movement. They were more capable of determining hand quality from these variables than when asked to estimate it directly.

The moral 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 lays in his hands.

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