"Compared to most other animals, humans are unusual in our tendency to help each other out. We donate to charity. We give blood. We show kindness to strangers, even when we stand to gain nothing in return. This behaviour is so odd that the natural question arises: are we alone in such selflessness? And if any animal could help to answer that question, it’s the chimpanzee, one of our closest relatives.

Dozens of scientists study the behaviour of chimps, looking at how these apes act towards their peers. But the results of these studies have been frustrating for many in the field. People who watch captive and wild chimps have documented hundreds of cases of seemingly altruistic behaviour. They have seen individuals helping each other to climb walls, consoling each other after fights, sharing food, risking death to save companions from drowning, and even adopting the babies of dead and unrelated peers. Anecdotes like these suggest that chimps, like humans, behave selflessly towards each other.

But experiments have often shown otherwise. In some studies, chimps choose to help their peers retrieve out-of-reach objects rather than doing nothing. But when chimps have a choice between two equal actions – say, cashing in a token that leads to personal gain versus another that also benefits a partner – they only looked out for themselves. One paper bore the title “Chimpanzees are indifferent to the welfare of unrelated group members”. Another concluded that “chimpanzees made their choices based solely on personal gain”.

Collectively, these studies championed a view of chimps as reluctant altruists, who only act selflessly in response to pressure, and who generally don’t help unfamiliar chimps, “even when they are able to do so at virtually no cost to themselves”. But Frans de Waal from the Living Links Centre at Emory University thinks that this portrait is wrong. He says, “The authors of these studies moved from not finding evidence for prosocial choice to thinking they had proven its absence.”

De Waal thinks that the previous tests handicapped the chimps by putting them in situations that masked their altruistic tendencies. They couldn’t communicate, they had to cope with complicated equipment involving levers, and they often sat so far apart that they had little understanding of how their choices affected their fellows. With his colleague Victoria Horner, de Waal designed a new experiment to account for these problems. And, lo and behold, chimps spontaneously helped their partners, even without any prompting..."

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