You have to go to bed earlier than usual. Your friend gets more ice cream in her cone, even though you asked for the same size. The whole class is punished for one student's outburst.

Every day, life delivers its little injustices. "That's not fair!" We've all said it at one time or another.

Now, a new study shows, some monkeys express the same sort of frustration. It's the first time scientists have found a sense of fairness in a species other than humans.

Researchers Sarah Brosnan and Frans de Waal of the Yerkes National Primate Research Center in Atlanta spent 2 years training brown capuchin monkeys how to trade rocks for food. She gave a monkey a rock and then held out her hand. If the monkey returned the rock, she'd offer food.

Then, Brosnan worked with five female monkeys from the original group who knew each other but were not related. She would accept a rock from one monkey and give it a yummy grape in return, while a second monkey watched the exchange. When the second monkey gave Brosnan a rock, she would hand it a less-tasty cucumber. After a while, the shortchanged monkey started either rejecting the cucumber or refusing to trade at all. In 10 out of 25 trials, it would nix the deal.

Next, Brosnan gave a grape to the first monkey without even asking for payment. In these cases, the second monkey bailed on the rock-for-cucumber trade 20 out of 25 times.

The study suggests that a sense of fairness is an important skill with adaptive value: It keeps us honest and helps us cooperate. In other words, we might be programmed to get upset if the neighborhood bully tries to steal our Hallowe'en candy.—E. Sohn