Humans are very, very bad at being random. In roshambo—aka rock, paper, scissors—this leads to probabilities and patterns that you can exploit to give your RPS opponent(s) severe and repeated thumpings. There are two ways to go about this: knowing the psychology and creating new psychology.
First, it’s useful to understand the generally held associations of the three symbols: Rock is solid, aggressive, and steadfast; scissors are aggressive in a more tactical way; and paper is flaccid, like shooting a very wet toilet paper spitball. Because of these (some claim), men are more likely to shoot rock, while women are more likely to shoot scissors. This also means that paper is habitually undershot. So shoot rock—it beats the feminine scissors, while remaining safe due to paper’s improbability.
But everyone knows this and so rock has the reputation as the choice of rookies.
If, before you started playing, you inserted into your explanation of RPS strategy the phrase “rock is for rookies,” you’d be priming your opponent not to shoot rock. Having done this, you could shoot scissors knowing it would win or tie. The crux is accurately evaluating your opponent’s skill level.
Now, a clever man would put the poison into his own goblet, because he would know that only a great fool would reach for what he was given. I am not a great fool, so I can clearly not choose the wine in front of you. But you must have known I was not a great fool, you would have counted on it, so I can clearly not choose the wine in front of me.
—Vizzini, The Princess Bride