Some of you may have read in the popular press about the study recently published in Science, by Felix Warneken and our old friend Michael Tomasello, in which both human infants (18 months) and young chimpanzees displayed altruistic behavior. The display of altruistic behavior is interesting, but what I find even more interesting is the level of theory of mind capabilities displayed by the young chimps. Here's how the experiments work (there are a few variations):
An experimenter is holding or trying to do something with an object (e.g., a marker or a sponge), while the infant or chimp is watching. At some point, the experimenter drops the object out of his reach (or the objects he needs to complete his task are out of reach), and makes a big deal of trying to reach it but failing. The infants and chimps then go to get the object and bring it back to the experimenter. If the experimenter drops the object in such a way that it was clear that he did it on purpose, however, neither the child or the chimp tended to help him retrieve it.
Unfortunately, the paper is only accessible with a subscription, so I can't link you to it (the reference is below, if you have a subscription or want to take a trip to the library). However, at Warneken's website, you can actually view clips from the experiments with both infants and chimps. So you can see how these things work.
Warneken, F. & Tomasello, M. (2006). Altruistic helping in human infants and young chimpanzees. Science, 311(5765), 1301-1303.