In contrast, the chimpanzees and capuchins treated the box competitively. Rather than instructing their offspring, mother chimpanzees snatched treats from them, and neither group seemed interested in learning from another's actions.
The conclusion: humans possess cumulative learning because of the way we help one another learn. "What sets us apart is the manner in which we humans work together to solve problems," Laland says. "We help each other, imitate each other, we give each other verbal instruction. These abilities seem to be critical to cultural learning."
Thus, we may owe our technical prowess and social sophistication to our outgoing natures and teaching skills. Kurzban, who co-authored a perspective piece in the same issue of Science, praises this experiment for its attention to some of the social and cognitive mechanisms that may influence human culture. "It foregrounds some of the most important psychological systems that influence human learning," he says.
Still, he cautions, the study's findings are principally correlative, and more experiments will be needed to examine the causes of cumulative learning. For example, preventing the children from speaking to one another could be a useful way to evaluate whether verbal communication gave the children an advantage over their fellow primates. Additionally, bridging the study of cumulative learning with the evolution of cumulative culture will require more complex investigation, and we are a long way from understanding how we evolved these capacities.
Kurzban also points out that other abilities may influence cumulative learning. For example, studies find that not only can children learn by imitation, as other animals do, but that they detect intention and act to achieve another's intended goal without relying on mimicry. "It's actually amazing because a person's intention is sort of invisible, and yet from a relatively young age kids can make inferences about invisible desire," Kurzban says.
The ability to recognize that other individuals have separate and discrete minds—or theory of mind—has also been attributed to nonhuman animals, and like culture the claim is contentious. Just this week a study published by theoretical biologists at the University of Groningen in the Netherlands, challenged a claim that the western scrub jay has this capacity. The jay—which caches food to save for later—will move and re-cache the same food item repeatedly when another bird is present. Some see this as evidence that the jay recognizes the other bird's intention to steal cached food and that the jay in turn understands that moving his stash will protect the treat. Using computer modeling, however, the Groningen biologists have demonstrated that the same behavioral outcome occurs if the jay simply caches food as part of its stress response. Like Occam's razor, when a simpler explanation for an animal's behavior exists, claims of culture or theory of mind are difficult to defend.
Yet other explanations persist: Primatologist Frans de Waal of Emory University has written extensively on the existence of culture and empathy in chimpanzees. He believes that the study comparing children, chimps and capuchins suggests humans are better equipped for certain forms of learning but does not rule out the possibility that the chimps and capuchins could be capable of cumulative learning.
"The absence of a capacity is far more difficult to demonstrate than its presence," de Waal says. He observes that although the puzzle-box experiment has successfully avoided some of the pitfalls of comparative studies, there are confounding factors, such as the children's ability to speak as well as the unequal value of rewards. The children cooperate and share stickers, which though prized have a largely symbolic value. In contrast, even a well-fed animal recognizes the value of food as something crucial to survival.