The Ultimatum Game
Why we reject free money to punish unfairness.
What it actually is
One player (the Proposer) gets a pot of money, say $100, and offers you a split. You (the Responder) do one thing: accept or reject. Accept, and you both keep the agreed shares. Reject, and you both get nothing. Pure self-interest says take any offer above zero, because $1 beats $0. But people routinely reject offers below about 20 to 30 percent, setting their own money on fire to deny a greedy stranger their $70. Fairness and spite are wired in deep enough that you will pay real cash to punish them.
Where you'll see it
Werner Guth ran the first version in 1982 at the University of Cologne, and the result has held up across decades and continents. When anthropologists took it global in the 1990s and 2000s, the Machiguenga of the Peruvian Amazon (a group with little anonymous trade) happily accepted lowball offers, while the Au and Gnau of Papua New Guinea rejected offers that were too generous, because in their gift culture a big present creates an obligation. Fairness is a real force, but the exact threshold is written by your culture, not your arithmetic.
Spot it in yourself
If you have ever refused a "fine" deal because the other side was getting way more and it made your skin crawl, you rejected free money to punish unfairness, and it felt like principle, not math.
The bias underneath
You silently anchor on a fair 50/50 split as the deal you deserve, so a lowball offer reads as a loss rather than a gain, and that loss stings hard enough that you torch real cash to avoid swallowing it.