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People are more likely to lend a hand to a perfect stranger if they have benefited from such kindness in the past. Now scientists have discovered that rodents show this behavior, too. A research team at the University of Bern in Switzerland trained rats to deliver food for one another by pulling a stick.

Then they divided the animals into two groups: some rats received food from other animals, whereas others did not. The team observed that rats that had ­received help were more likely to pull the stick for unfamiliar animals—going one step beyond the well-documented “you scratch my back, I’ll scratch yours” reciprocity that is seen in many species.

Studies have demonstrated this kind of altruism toward strangers—termed generalized reciprocity—in humans. For example, one experiment showed that people who found money in a telephone booth were more likely to help a stranger pick up dropped papers. But scientists have not yet figured out whether cultural experience or natural selection explains such acts of kindness. The fact that rats show generalized reciprocity hints that an evolutionary mechanism is involved, the researchers say.

Scientific American: Pay It Forward: Animals show altruism toward strangers