Zero-Sum Games
When your win must be someone's loss, and when it isn't.
What it actually is
A zero-sum game is one where the total never changes. Every dollar you gain is a dollar someone else lost, so the payoffs always sum to zero and cooperation is impossible by definition. Poker is zero-sum. Splitting a fixed inheritance is zero-sum. The trap is that most of real life is not zero-sum, it is variable-sum: two companies signing a contract, two countries trading, two people negotiating a salary, all of these can leave both sides richer or both sides poorer. The mistake that ruins deals is treating a growing-pie situation like a fixed-pie one, fighting over slices while the whole thing could have gotten bigger.
Where you'll see it
In 1978 an Egyptian negotiator and an Israeli negotiator were deadlocked over the Sinai at Camp David, each demanding the same land. It looked purely zero-sum until someone asked why each side wanted it. Egypt wanted sovereignty, Israel wanted security from tanks on its border. The deal: Egypt got the land back, Israel got it demilitarized, and both walked away with the thing they actually cared about. The pie was never fixed, they had just assumed it was.
Spot it in yourself
You catch yourself thinking "if they're happy with this deal, I must have left money on the table." That reflex is you assuming a fixed pie, and it makes you fight over crumbs instead of asking what the other side values that costs you nothing.
The bias underneath
Whether a negotiation looks win-lose or win-win is decided by how you frame it, not by the underlying reality, and a fixed-pie frame makes you fight over slices instead of growing the pie.