Is selfless behavior in humans a unique evolutionary development? For years, studies designed to test prosociality, or altruism, in chimpanzees have presented them as "reluctant altruists" that are indifferent to the welfare of their fellow chimps. But now it seems that chimps care more for their fellow apes than we realized.

A new study has shown that chimpanzees demonstrate spontaneous generosity, challenging the popular notion that humans developed altruistic tendencies only after our evolutionary split with apes.


In a Prosocial Choice Test (PCT), a chimp must choose between performing a prosocial action that rewards itself and a partner, and a selfish action that benefits only itself. Four such studies have been conducted on chimps in the past, and in all four studies chimps have failed to demonstrate a prosocial preference.

The observation of prosocial behavior under more natural circumstances (in the wild, for example) led researchers Victoria Horner and Frans de Waal at the Yerkes National Primate Research Center to question whether chimpanzees had not demonstrated prosocial behavior in prior studies due to flaws in experimental design. In their research paper, which is published online in this week's Proceedings of the National Academy of Sciences, Drs. Horner and de Waal cite several factors that could explain the negative findings of past studies, including:

...the complexity of the apparatus used to deliver rewards, the actors' preoccupation with visible reward options, limited communication between actors and participants, and competitive attitudes by actors toward the partners.

To account for these complicating factors, the researchers designed a new, more straightforward testing procedure. In the new test, the chimp being observed is provided a bucket of 30 tokens that they can trade with a human test proctor in exchange for a reward. Fifteen of the tokens are one color and result in a "selfish" outcome, and fifteen tokens are another color that result in a "prosocial" outcome.


If the chimp selects a selfish token, the experimenter presents it with a food reward. If the chimp selects a prosocial token, however, the experimenter distributes two rewards: one to the chimp that selected the coin, and the other to a second chimp positioned opposite a transparent barrier from the first chimp. Food rewards were wrapped in paper to reduce the probability that the chimps would be distracted by visible food.

Shown here is a schematic of two chimpanzees in a typical test setting. The authors describe the scene as follows:

While her partner (Left) watches through a mesh partition, the actor (Right) reaches into a bucket with 30 tokens, 15 of each color, to select one and hand it to the experimenter. The token then is placed in full view, after which, depending on the token choice, one or two paper-wrapped rewards are held up in the air. A reward is handed either to the actor or to both chimpanzees.

Every single one of the chimpanzees tested — seven in total — demonstrated an overwhelming preference for the prosocial choice. What's more, the chimpanzees doing the choosing proved to be especially altruistic towards their partners when the latter would either wait patiently or gently remind the choosing chimp of their presence. More demanding chimps, on the other hand, who would beg persistently or spit water on the chimp doing the choosing, were less likely to be rewarded by their partner's choices. Translation? Altruism on the part of the choosing chimp was spontaneous, and was performed independently from direct requests, pressure, and intimidation.

"We were excited to find female after female choose the option that gave both her and her partner food," said Dr. Horner. "It was also interesting to me that being overly persistent did not go down well with the choosers. It was far more productive for partners to be calm and remind the choosers they were there from time to time," she continues.

According to the authors, this study provides definitive evidence that chimpanzees are prosocial animals even in well-controlled experiments, but it's just as important for what it says about us.

The tendency for humans to help others, even in situations where they derive no direct benefit from their actions, has been well documented. In the past, the failure of chimps to engage in prosocial behavior strongly suggested that humanity's tendency to engage in selfless behavior was unique to our species. Given these recent findings, however, it looks as though our altruistic tendencies may, in fact, be rooted deep in our evolutionary history.


Read the original research paper via PNAS
To learn more about their Chimpanzee research, visit the labs of Drs. Victoria Horner and Frans de Waal

Testing schematic generously provided by Yerkes National Primate Research Center
Image of sharing chimps via