When all else is equal, human children prefer to work together in solving a problem rather than on their own. Chimpanzees, on the other hand, show no such preference. That's according to a study of 3-year-old German kindergarteners and semi-free-ranging chimpanzees reported online on October 13 in Current Biology, a Cell Press publication.

"A preference to do things together instead of alone differentiates humans from one of our closely related primate cousins," said Daniel Haun of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

"Once we know the underlying motivations of this tendency, we will have learned something new about human nature that differentiates it from chimpanzee nature."

Human societies are built on collaboration, the researchers explained. From a young age, children will recognize the need for help, actively recruit collaborators, make agreements on how to proceed, and recognize the roles of their peers to ensure success.

Chimpanzees are cooperative, too, working together in border patrols and group hunting, for instance. Still, the researchers thought humans might have greater motivation to cooperate than chimpanzees do.

"We expected differences between human and chimpanzee cooperation, because humans cooperate in a larger variety of contexts and in more complex forms than chimpanzees," Haun said.

Haun, along with Yvonne Rekers and Michael Tomasello, presented 3-year-old German children and chimpanzees living in a Congo Republic sanctuary with a task that they could perform on their own or with a partner.

Specifically, they could either pull two ends of a rope themselves in order to get a food reward or they could pull one end while a companion pulled the other. The task was carefully controlled to ensure that there were no obvious incentives for the children or chimpanzees to choose one strategy over the other.

"In such a highly controlled situation, children showed a preference to cooperate; chimpanzees did not," Haun said.

The children cooperated more than 78 percent of the time, compared to about 58 percent for the chimpanzees. Those statistics showed that the children actively chose to work together, whereas chimps chose between their two options at random.

Haun says the findings suggest that behavioral differences between humans and other species might be rooted in apparently small motivational differences.

Future work should compare cooperative motivation across primate species in an effort to reconstruct the evolutionary history of the trait, the researchers say.

"Especially interesting would be other cooperative-breeding primates or one of our other close phylogenetic relatives, the bonobos, which have both previously been argued to closely match some of the human prosocial motivations," they wrote.