From The Huffington Post:
The dogs sat side-by-side with an experimenter in front of them. In front of the experimenter was a divided food bowl with pieces of sausage on one side and brown bread on the other.
The dogs were asked to shake hands and each could see what reward the other received.
When one dog got a reward and the other didn't, the unrewarded animal stopped playing.
When both got a reward all was well.
One thing that did surprise the researchers was that, unlike primates, the dogs didn't seem to care whether the reward was sausage or bread.That last part is hilarious.
Moral: treat all dogs the same. OR ELSE. They will turn their heads away from you!