Monkeys taught to use money

The authors of Freakonomics : A Rogue Economist Explores the Hidden Side of Everything (a brilliant, though somewhat repetitive book about how empirical measurement and statistical analysis suggest that many of the causal relationships in our society aren't what they seem) report on an economic experiment to teach monkeys to use money. Faced with financial realities, the monkeys act pretty much like people:
A capuchin monkey must decide how to spend his budget of twelve coins (located on the yellow and white striped tray in the front of the trading room.) Two human research assistants are present (one wearing blue and one wearing red), and both hold a piece of food in an orange dish for the monkey to see. The red research assistant "sells" grapes and the blue research assistant "sells" Jell-o cubes, with each piece of food costing a coin from the monkey's budget.

The capuchin must make a decision analogous to a grocery-store shopper's: how much of their budget to spend of grapes and how much to spend on Jell-o...

Two unrelated tamarin monkeys sit adjoining each other. Every fifteen seconds, a human research assistant brings a tray loaded with a marshmallow that is just out of reach of one of the monkeys. The only way that monkey can eat the treat is if his partner pulls a red handle that will put the marshmallow within reach.

But pulling the handle doesn't bring any reward for the puller: pulling the handle only pays if the monkey who receives the marshmallow reciprocates by pulling in the future. Like many human situations, the monkeys must work together to obtain food and build trust while punishing failures to cooperate.

Link (Thanks, Brad!)