But rock-paper-scissors is rarely played by robots. More often, it's played by humans -- humans that operate emotionally, even irrationally. And humans playing rock-paper-scissors, as researchers from Zhejiang University in China recently showed, often follow a predictable pattern -- the "win-stay lose-shift" strategy.
Researchers recruited several hundred students to play in a tournament of rock-paper-scissors featuring cash prizes (so as to incentivize genuine competition). In studying the games, strategies and patterns, researchers noticed that after a contestant won a round, they were much more likely to stick with the same symbol, whereas losers were more likely to switch to one of the other two.
In theory, a good player should employ game theory: randomizing their choices to remain unpredictable. Two players employing game theory tactics would essentially cancel each other out, each with equal probability to win -- a reality known as the Nash equilibrium.
But, as the researchers found out, humans often fail to employ such tactics.
"The game of rock-paper-scissors exhibits collective cyclic motions which cannot be understood by the Nash equilibrium concept," explained researchers.
"Whether conditional response is a basic decision-making mechanism of the human brain or just a consequence of more fundamental neural mechanisms is a challenging question for future studies," the study's authors added.