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Kevin Holmes’ Research: Eyes Betray Blackjack Players

A study by Colorado College Assistant Professor of Psychology Kevin Holmes has the potential to increase one's chances of winning at the blackjack table.

Holmes and colleagues at Emory University found that blackjack players with high-value cards tend to glance to the right while players with low-value cards unconsciously look to the left. The study, titled "Gamble on gaze: Eye movements reflect the numerical value of blackjack hands," was published in the Psychonomic Bulletin & Review and already has been picked up by media in Australia and the UK.

Research suggests that people produce involuntary tell-tale behaviors while doing arithmetic silently in their heads. For example, when pointing to an arithmetic solution on a visually presented number line, participants are biased leftward on subtraction problems and rightward on addition problems.

"We wanted to find out whether people rely on this so-called 'mental number line' when doing arithmetic in more of a real-life setting," says Holmes.

To investigate this, Holmes and his colleagues, Vladislav Ayzenberg and Stella Lourenco, conducted two experiments using the popular card game blackjack. In blackjack, players must decide whether to keep drawing cards to increase the value of their hand to 21 - but not go above that, which results in a loss. Players must mentally add the values of successive cards, continually keeping in mind the total value of their hand.

In the two experiments, 58 participants played a computerized version of blackjack. New playing cards were presented to them in succession in the middle of a screen. Participants then had to decide whether to take another card or keep their hand as is, which requires mentally adding the values of the cards they already have. Holmes' team analyzed in which direction participants spontaneously moved their eyes while they were doing this mental math.

"We found that participants' eye movements along the horizontal axis reflected the total value of their cards," says Holmes. Participants tended to look leftward on the screen when they had smaller-value hands and rightward when they had larger-value hands. These effects were driven by the total value of the cards in the player's hand, not merely by the number of cards the player was holding or the value of the card just dealt.

"Our research suggests that blackjack players may unwittingly give away the value of their hand through their eye movements," says Holmes. "Whether our findings will help blackjack players in real life remains to be investigated."

Report an issue - Last updated: 12/16/2020