A sense of fair play is uniquely human and is shaped not only by social forces but by heredity, according to a new study involving chimps and separate study testing identical human twins.

In a food-sharing experiment published Friday in the journal Science, chimps readily accepted stingy offers no human would take, suggesting the human sense of fairness evolved to foster cooperation in a complex society made up of unrelated individuals and groups.

“In the context of everyday life, it is an advantage to not allow people to treat you unfairly. If you do, they will roll over you,” said Bjorn W. Wallace, an economist with the Stockholm School of Economics and lead author of one of the studies.

The chimp study, conducted by researchers at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, used the “ultimatum game,” a classic test used to explore fairness. A typical game is played by two people. One of them is told to divide a small amount of money. If the second player accepts the offer, the money is shared. But if the second player rejects the proposal because it is not generous enough, the players receive nothing.

In general, offers that give the second player less than 20 percent of the money are rejected, giving the first player a strong incentive to be fair.

Researchers modified the game for chimps, using raisins as a reward. Two chimps were separated behind a wire mesh through which they could view two trays holding a total of 10 raisins divided in different ways.

The first chimp offered one of the trays to the second chimp by using a rope to pull the tray almost within reach. If the second chimp liked the offer, it pulled a rod to bring the tray close enough to grab the raisins. If the second chimp did not like the way the raisins were divided, it refused to pull out the tray and neither chimp received raisins.

The experiment was repeated with two trays containing varying combinations of raisins.

The chimps, in contrast with humans in previous studies, tended to accept any offer and didn’t become upset when they were offered a small amount of raisins or none at all.

Lead author Keith Jensen of the Max Planck Institute said the chimps behaved more rationally than people because “it makes perfect economic sense to accept any nonzero offer and to offer the smallest amount possible while keeping the most for yourself.”

Human sensitivity to fairness might have evolved along with empathy and other traits that allow individuals to cooperate, Jensen said. Groups of cooperative individuals would have competitive advantages over groups whose members don’t cooperate, he said. To get along, people need to have some degree of concern for others.

Jensen, however, cautioned that the origins of fairness “are very speculative and debatable.”

Sarah Boysen, a psychologist who studies animal behavior at Ohio State University and was not connected with the study, warned against over-interpreting the experiment. Chimps have a strong sense of justice, she said, but it is just not the same as humans.

“Deviations from their code of conduct are dealt with swiftly and succinctly, and then everybody moves on,” she said of chimps. “They’re more adaptive than we are - just look at the Middle East.”

In the second study, published in the Proceedings of the National Academy of Sciences Tuesday, researchers from Sweden and the United States showed identical twins tended to use the same strategy when playing the “ultimatum game,” suggesting our sense of fairness is shaped by heredity.

In a variation of the game, researchers proposed 11 ways to divide the equivalent of $15 and asked 71 pairs of fraternal twins and 258 pairs of identical twins to record which offers the would accept or reject. Offers ranged from the entire amount to nothing.

Because identical twins share all the same genes but fraternal twins do not, the study could detect genetic influences in how participants played the game while controlling for some environmental factors, such as upbringing within their families.

Researchers then looked at each set of twins individually to see how closely their rejection decisions matched. They found there was no correlation among fraternal twins but a 42 percent likelihood that identical twins would make the same choice.

The results mean many personal economic choices, such as whether to save money or spend it, might be substantially influenced by heredity, said Wallace, who conducted the research with colleagues from Sweden’s Karolinska Institutet and the Massachusetts Institute of Technology.

However, environmental factors were still important and relationships not shared by the twins, such a friends and jobs, had a greater influence on decisions than genetics in the study, Wallace said.