The first piece of research discussed in this post by Anil Dash proves nothing about our rationality except that economists typically harbour the dubious notion that the idea of money that we carry around in our heads is purely quantitative.
In fact it's highly unlikely that we would manage our budgets like a mental spreadsheet - there are different categories of wants and different emotional contexts to purchases. (For instance, the effort we put into buying a new TV may exceed that we're prepared to allocate to a similarly-priced but more regular, overhead-type cost.)
In the second study involving the Capuchin monkeys it's unclear whether the subjects understood the gambling game. Otherwise, of course the monkey that has two grapes will object to losing one? The mathematics of survival are brutally simple and selfish. As for us humans, there's a gameshow format in there somwhere.