Fairness
In the ultimatum game, 2 players, a proposer and a responder, divide a reward. The responder can either accept the proposer’s division or reject it. If he rejects it, both players receive nothing.
Scores of studies have run the ultimatum game across cultures and ages. Universally, people reject any share lower than 20% — apparently to punish the greed of the proposer.
A study by Björn Wallace, et al (”Heritability of ultimatum game responder behavior“), suggests that the sense of fairness is rooted in genetics.
Wallace played the ultimatum game with twins. He neutralized the effect of upbringing and exposed that of genetics by comparing identical twins (who share all their genes) with fraternal twins (who share half).
Each twin of a pair played the ultimatum game, both as proposer and as responder. In the case of identical twins, there was a striking correlation between the average division that each member of a pair proposed and also between what they were willing to accept. In other words, their senses of what was fair were similar. No such correlations were seen in the behaviour of fraternal twins.
“Patience, fairness and the human condition,” The Economist