Prisoner's dilemma
Also said as Iterated prisoner's dilemma, Tit for tat, Cooperate or defect
Two players each pick cooperate or betray. Betraying is always better for you whatever the other does, yet both end up worse off than if both had cooperated. Play it many times and things change, because betrayal can then be punished.
It explains why players grief, hoard and betray when the rules reward it, however nice they are. If you want co-operation in a game, the rules have to make the relationship last: reputation, repeated meetings, a future.
Say it
“Players keep killing each other at the shared stash. Make it repeat: let squads stay together for several raids and show past betrayals, so that stealing has a price next time.”
Where you’ve seen it
The British game show Golden Balls ended with the actual dilemma, Split or Steal. In the 1980 tournaments run by Robert Axelrod, the winner for repeated play was Anatol Rapoport's Tit for Tat: start nice, then copy whatever the other did last.
Not to be confused with zero-sum game
In a zero-sum game, my gain is your loss, and co-operation is meaningless. In a prisoner's dilemma both can win or both can lose, which is why betrayal is such a temptation.