Some really smart scientists have developed experiments where 2 players were expected to share a sum of money. In the most basic form of the game, the first player is allowed to choose a portion of the total pot of cash for himself. Let's say there is $10 on the table and he chooses to keep $6. The remaining money goes to the second player, who can choose to accept the remaining $4 or reject it. If the second player rejects the money, neither player takes home any cash. If the second player accepts the money, both players go home a little richer. A rational second player should accept any amount of cash because no matter what, she makes some money in the deal... Even if it is only 1 dollar or 1 cent. But this isn't what happens. If the amount taken by the first player seems unfair, the second player will punish the first player by rejecting the deal, even though she is punishing herself as well. Human pride and a sense of fairness clouds our judgement. This experiment demonstrates how reciprocity directly affects human behavior. Humans like to retaliate.