That's not a poker face, this is a poker face

What does your poker face look like? If it's the traditional, stern, emotionless expression, you may want to consider practising a new one. Erik Schlicht and colleagues report that a friendly, trustworthy face is more likely to influence your opponents, leading them to think that you've got a good hand - that you're not bluffing.

Schlicht's team had 14 relative novices play hundreds of one-shot rounds of a simplified version of Texas Hold'em poker against hundreds of different 'opponents'. Each round the participants received a two-card hand and their opponent had bet 5000 chips. They had to decide whether to 'fold' or 'call'. Folding meant they would lose 100 chips guaranteed. By calling, they would win 5000 chips if their hand was stronger then their opponent's, or lose the same amount if their hand was weaker. To boost their motivation, participants had the chance to win a small amount of money based on the outcome of one randomly chosen hand out of the 300 that they played.

Each round, before making their decision, the participants saw a picture of their opponent's face. These were morphed to appear either untrustworthy, neutral or trustworthy (see picture). Participants were told that, as in real poker, the different opponents could have different styles of play (but no mention was made of faces providing a clue to style).

Because participants played just one round against each opponent there was no opportunity to use past behaviour to make judgments about their style. This meant the only information participants had to go on was the cards in their own hand and any inferences they'd made about their current opponent's playing style based on his face. They didn't receive any feedback during play on whether they'd won a round or not.

On each round, there was an optimal decision for participants to make considering the cards in their hand and the stakes involved in holding or calling. The researchers were careful to ensure that participants' hands were of equal value across the different categories of opponent face - trustworthy, neutral, untrustworthy. Unbeknown to the participants, their opponents' hands bore no relation to their facial expression.

The key finding was that faces with neutral or untrustworthy expressions made no difference to the decisions the participants made. By contrast, if an opponent had a trustworthy face, the participants took longer to decide what to do and they made less optimal decisions. Effectively, they were behaving as if their opponent had a better hand.

'Contrary to the popular belief that the optimal face is neutral in appearance,' the researchers said, 'poker players who bluff frequently may actually benefit from appearing trustworthy, since the natural tendency seems to be inferring that a trustworthy-looking player bluffs less.' Before you try this out at your local poker den, remember the findings apply when you're up against new opposition and there's little other information to go on.
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Schlicht EJ, Shimojo S, Camerer CF, Battaglia P, & Nakayama K (2010). Human wagering behavior depends on opponents' faces. PloS one, 5 (7) PMID: 20657772

For further info, lead author Erik Schlicht has created a webpage where he answers frequently asked questions about this research.