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For all its apparent simplicity, a new experimental study showing that mouse-like rodents can be nice to each other now stands at the vanguard of a scientific revolution. The paper, published in the journal Science and promoted under the headline, “Empathy More Common in Animals than Thought,” could never have been published in the late 20th century, said University of Michigan psychologist Stephanie Preston. Not only would it have been rejected, she said, it would have been ridiculed.

The paper would have violated a long-standing prohibition against anthropomorphism — the attribution of human motives or feelings to animals. This taboo made some sense, in that scientists risked clouding their careful observations if they projected their own feelings or motives onto animals. But in recoiling from anthropomorphism, biology warmed up to an opposing assumption — that nonhuman animals had no emotions, no feelings and no inner lives.

Now, scientists are starting to question this long-standing belief. The subject of the new experiment is the prairie vole, a social, monogamous creature native to the North American Midwest. Both sexes care for offspring. When researchers subjected one member of a mated pair to an electric shock, the unharmed vole groomed its distressed mate for as much as 10 minutes. Prairie voles primarily comforted family members this way — engaging less in consoling behavior toward strangers.

It’s not the first time scientists have observed rodents showing kindness. A famous 2012 experiment suggested that even rats could be generous. Experimenters created a tiny enclosure — the size of a rat coffin — that another rat could learn to open from the outside. Though the enclosure took some effort to open, rats more often than not freed trapped companions — even if it meant delaying a treat of chocolate chips and then having to share it.

Scientists still disagree over whether any of these rodent behaviors are motivated by empathy. That’s because they don’t agree on the definition of empathy, said biologist Frans de Waal, one of the authors of the vole paper and a leader in the study of animal social behavior.

He said some scientists define empathy as a form of thinking, requiring an ability to put oneself in the shoes of another person or, say, vole. Researchers sometimes refer to this interpretation as theory of mind. “This requires a very high level of cognition,” de Waal said. “Obviously a rodent couldn’t get there.”

Nor could some humans, however. Infants, de Waal said, will cry when they hear other infants crying. Such emotional contagion is something people share with some other mammals. Humans can respond to someone in distress before understanding what’s wrong. On the other hand, he said, understanding another person’s mind-set without a grasp of emotion is associated with psychopaths.

The Science paper demonstrated that physiologically, people and voles react in similar ways to the distress of others. Humans show activation in the same parts of the brain and changes in the same hormones — especially oxytocin, which is associated with maternal care and other forms of bonding among mammals.

Why are we just discovering this now? We’re not, said Preston. Studies from the 1950s and 1960s showed rats, pigeons and monkeys comforting or helping each other. They were ignored as unscientific. She said that what’s happened in the 21st century is a philosophical shift away from the abhorrence of anthropomorphism.

Underlying the shift is a scientific preference for simple, elegant explanations over convoluted ones, said Robert Jones, a philosopher at California State University at Chico. One manifestation of this preference is Occam’s Razor, the principle that the best argument is the one with the fewest underlying assumptions.

What we know is that living things are related through evolution, and that humans, voles and those chocolate-sharing rats are all part of the same mammalian branch of the family. In addition, scientists have observed similar neural and hormonal changes associated with social behavior in humans and other mammals. If an experiment shows voles behaving as if they’re showing empathy, Jones said, you have to jump through a lot of hoops to explain that behavior as a different, unrelated phenomenon.

De Waal said there’s also a softening of the line scientists used to draw between altruistic and selfish behavior (with altruism reserved only for humans). In real life, he said, the selfish and the selfless are intertwined. We help others and become good members of our families and communities and are repaid in social support.

Humans are exceptional in their problem solving abilities, use of language, artistry and technology. But the traits we most value in fellow human beings may not necessarily coincide with the traits that set us apart from other creatures.

Faye Flam writes about science, mathematics and medicine.

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