Two articles:

STUDIES SHOW DOGS HAVE SENSE OF FAIRNESS

Press, Randolph E. Schmid, November 8, 2008

No fair! What parent hasn't heard that from a child who thinks another youngster got more of something? Well, it turns out dogs can react the same way. Ask them to do a trick and they'll give it a try. For a reward, sausage say, they'll happily keep at it. But if one dog gets no reward, and then sees another get sausage for doing the same trick, just try to get the first one to do it again. Indeed, he may even turn away and refuse to look at you.

Dogs, like people and monkeys, seem to have a sense of fairness.

more at: http://tinyurl.com/6ao8xv

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DOGS UNDERSTAND FAIRNESS, GET JEALOUS, STUDY FINDS

Morning Edition (National Public Radio), Nell Greenfieldboyce, December 9, 2008

Dogs have an intuitive understanding of fair play and become resentful if they feel that another dog is getting a better deal, a new study has found.

more at:

http://www.npr.org/templates/story/story.php?storyId=97944783