Playing rock-paper-scissors to decide who gets the next round of beers at Wednesday night bowling? It's not as random as you may think; scientists say there's a way to win almost every time.
Researchers at a Chinese university say closely watching how opponents choose to keep or change their strategy during multiple rounds in the game can give you a slight edge -- perhaps enough to keep your beer money in your pocket.
People tend to pick equally among the three choices -- rock, paper or scissors -- so the outcome of the initial round is truly down to chance.
But if it's "two out of three" to see who heads for the bar, some simple rules of how opponents move through the possible three choices in subsequent rounds can give you an advantage, the researchers at Zhejiang University found.
A player who wins will usually stay with his same choice.
A player who loses a round will normally move to another choice in what the researchers termed a "clockwise direction" -- from rock to paper, from paper to scissors, and from scissors to rock and around again.
The researchers called this strategy "win-stay, lose-shift," and knowing most players fall into that can improve your chances.
So if you're the loser, the researchers say, make the choice that will beat the choice just played by your opponent, because he's likely to play it again. If you were the winner, don't make the same choice; rather choose the item that would have beaten what you previously played, because your opponent is likely to move to that choice.
It's all down to game theory and one of its tenets known as the "Nash equilibrium," which holds that people will switch equally over the three options.
But over a longer period of play, many players will fall into the "win-stay, lose-shift" cycle, which a wily opponent can take advantage of, the researchers say.
The patterns were seen to emerge in the study that recruited 360 students to play 300 rounds of the game.
Rock-paper-scissors or some variation of it is believed to have been around as a decision-making game for more than 2,000 years.
In a Chinese version from around 200 B.C., a frog -- represented by displaying a thumb, would contest a slug (the little finger) or a snake (the index finger.)