• Updated
24 September 2020

Evolutionary origins of money categorization and exchange: an experimental investigation in tufted capuchin monkeys

In human modern societies, money is universally recognized as the most efficient medium to acquire goods and services and its use allowed the transition from barter to our modern economic system. Money power is so pervasive in our societies that it is crucial to understand what are the biological underpinnings of human attraction to money. 

A new series of studies, in collaboration with the Institute of Cognitive Sciences and Technologies of the CNR of Rome, Italy and the Institute Jean Nicod of the CNRS of Paris, investigated whether some features of money may be traced back to the exchange habits of capuchin monkeys, a South-American primate species whose lineage split from ours more than 35 million years ago. 

"There are many evidences that some non-human primates, like chimpanzees and capuchin monkeys, can flexibly exchange tokens with a human experimenter to obtain a food reward. They can also attribute different values to different tokens and perform flexible computations to maximize their payoff when presented with binary choices between various amounts and combinations of different types of tokens" says the study’s lead author Francesca De Petrillo, a comparative psychologist based at the Institute for Advanced Study in Toulouse, France. "Therefore, we were wondering whether capuchin monkeys, as humans, were able to categorize and exchange tokens according to their current validity and familiarity”.

De Petrillo and her co-authors presented six capuchin monkeys, housed at the Primate Center of the Unit of Cognitive Primatology of the ISTC- CNR in Rome, with two token-trading experiments. "This colony of capuchins is well-habituated to exchange tokens with humans and they have been part of many study investigating their trading abilities" explains Elsa Addessi, researcher of the ISTC-CNR of Rome "In this study we provided each monkey with a set of four different tokens (i) valid/familiar tokens, used in previous experiments, leading to a food reward, (ii) valid/unfamiliar tokens, introduced in the present study, leading to a food reward, (iii) invalid tokens, used in previous experiments, but here lost exchange value, and (iv) no-value items, introduced in the present study, with no exchange value. Then we measured which tokens they exchanged with the experimenter and in which order".

The researchers found that capuchin monkeys readily recognized the validity of tokens as medium of exchange regardless of their familiarity, as did human subjects with money, by consistently exchanging a higher number of valid tokens than invalid tokens or no-value items. Moreover, they also found that capuchins were able to engage in chains of reverse and direct exchanges. In a second experiment, researchers gave capuchins a piece of food that could be exchanged for a token that, in turn, could be exchanged for a higher quality food. Capuchins successfully performed the exchanges, but only when the first food was of very low quality compared to the higher quality food. 

"Our study showed for the first time that a non-human primate species understands which features tokens must have to function effectively, as shown in humans with coins" says Francesca De Petrillo "This suggests that studying the trading abilities of non-human primates is crucial to understand the evolutionary origins of our complex monetary system".

"The fact that capuchins were able to exchange food for a token points to proto-monetary behavior since the exchange is mediated through an item that is in principle a primary reinforcer and temporarily becomes a secondary reinforcer" concludes Sacha Bourgeois-Gironde, an economist at the Institute Jean Nicod of the CNRS of Paris "The transition from primary to secondary reinforcers was essential for the emergence of commodity money in our species". 

Paper reference: De Petrillo, F., Caroli, M., Gori, E., Micucci, A., Gastaldi, S., Bourgeois- Gironde, S., & Addessi, E. (2019). Evolutionary origins of money categorization and exchange: an experimental investigation in tufted capuchin monkeys (Sapajus spp.). Animal Cognition, 1- 18.

IAST Press release