In the classic experiment, one player splits a pot and the other can accept (both keep their shares) or reject (both get nothing). Rationally any offer beats zero — yet people routinely reject lowball offers, paying to punish unfairness.
The principle: Fairness isn't a soft nicety; it's load-bearing. Offer a deal that's efficient but feels unjust and people will torch the whole thing to deny you the win. Price the insult, not just the economics.