The Prisoner's Dilemma
Two rational players, both worse off, the math of why trust is hard.
What it actually is
Two players each pick "cooperate" or "defect" without knowing what the other chose. The trap: no matter what your partner does, defecting pays you more individually, so a "rational" player always defects. But when you both run that logic, you both defect and both end up worse off than if you had both cooperated. That gap between the smart-for-me move and the good-for-us move is the entire game. Individual rationality and group rationality point in opposite directions, and the math guarantees it. Repeat the game enough times, though, and cooperation claws its way back, because now your partner can punish you tomorrow for defecting today.
Where you'll see it
OPEC lives this game every day. Every member is richer if everyone limits oil output to keep prices high, and every member is individually tempted to quietly pump extra and pocket the cash. So quota-cheating is chronic, and prices crater exactly when the cartel most needs discipline. The 1980s Coke versus Pepsi ad wars were the same trap: both would have made more money spending less, but neither dared cut its budget first, so they burned billions defecting because the other might.
Spot it in yourself
Any time you catch yourself thinking "I'd cooperate, but they'd just take advantage," you are already defecting in advance. You have locked in the worse outcome for both of you before the other person even moved.
The bias underneath
You defect not to maximize the joint payoff but because the fear of being the lone sucker looms far larger than the shared gain from mutual trust.