Earlier this month, TED posted an excerpt from a recent talk by Dr. Frans de Waal, a Dutch primatologist and ethologist. He has in the past performed a number of studies examining primate social behavior, particularly regarding empathy and cooperation.
In the talk, Dr. de Waal discusses an experiment conducted about ten years ago. It involved Capuchin monkeys; he describes that the monkeys were familiar with one another and lived in a group together. Two monkeys were placed in side-by-side cages and asked to perform a very simple task. When they were rewarded with the same food (in this case, cucumbers), they were content to perform the task repeatedly.
They decided to introduce inequity by rewarding one monkey with a grape and the other with a bit of cucumber. The monkey is initially content with receiving the cucumber, but isn't as pleased after seeing that her partner receives a much better reward.
There are a number of parallels that could be made (the comments on YouTube seem to focus heavily on the disparity between the 1% that owns over 70% of all financial assets and the remaining 99%), but I thought that an easier comparison could be made by examining the difference in pay that a man and woman receive in the same job within the same company with similar work experience and training.