Cam sex economics
The tamarins were fairly cooperative but still showed a healthy amount of self-interest: over repeated encounters with fellow monkeys, the typical tamarin pulled the lever about 40 percent of the time. They conditioned one tamarin to always pull the lever (thus creating an altruistic stooge) and another to never pull the lever (thus creating a selfish jerk).
It took several months of rudimentary repetition to teach the monkeys that these tokens were valuable as a means of exchange for a treat and would be similarly valuable the next day.
Having gained that understanding, a capuchin would then be presented with 12 tokens on a tray and have to decide how many to surrender for, say, Jell-O cubes versus grapes.
"The capuchin has a small brain, and it's pretty much focused on food and sex," says Keith Chen, a Yale economist who, along with Laurie Santos, a psychologist, is exploiting these natural desires -- well, the desire for food at least -- to teach the capuchins to buy grapes, apples and Jell-O.
"You should really think of a capuchin as a bottomless stomach of want," Chen says.
In economist-speak, the capuchins adhered to the rules of utility maximization and price theory: when the price of something falls, people tend to buy more of it.