Here is an interesting study on TED:
Now, some personal comments:
There is a slight difference between the 'coin' insertion in human society and the one done in this study. The coins appeared as a development of the barter system, because it got difficult to transport and exchange merchandise for other merchandise. In the the monkeys case, there was no economic development, the currency was artificially introduced into their existence. So they still function on a barter system: the give away their useless chunk of metal in order to get very useful food. And they know that more trinkets mean more food so they steel the coins from others; this only proves the slick savage side in humans...
Also, they don't save up the coins; well, this option was never revealed to them, every time, one shinny disc equals food; this is also a false comparison with our society, because we save up money but do bad investments...This thing could only show is that there are no scientist monkeys to wonder what it would happen if 2 coins would be given away.
Last thing, about the gain/loss study, it is purely psychological, in one case there is a chance of winning nothing which we want to avoid and in the other one of losing nothing, which is more appealing. Faced with the 2 situations at once we would probably admit that they equivalent, but something inside makes us act in one way or the other. For the monkeys this thing is more visual, but I guess it resides in the same behavior pattern as us, only difference is that the monkeys can't evaluate probabilities...you will surely say that this was one of the aspects of the study; true, but still, I wanted to say this and to add that with statistics anything can be proved all that matters is how the topic is approached.
All in all, we have residual instincts which need to be channeled in precise directions and not to be left to wander and take control of purely rational decisions.