It is considered one of our most human characteristics, but new research suggests empathy may not be so unique to us after all.
In fact, one of the most feared and hated creatures in the animal kingdom could share this beneficial trait: rats.
In a study published Thursday in the journal Current Biology, researchers show that rats don’t want to hurt other rats — a human-like feature known as harm aversion.
Harm aversion is what makes humans feel bad when they hurt others' feelings or otherwise cause others pain. It is an essential ingredient in human cooperation, enabling us to work together for the greater good of our species.
But scientists know very little is known about what actually happens in our brain to make us averse to harming others. But the evidence so far suggests that we harm other people, our brains are hard-wired to share some of the pain we inflict.
"Like humans, [rats are] actually trying to avoid harm to other individuals.”
Finding the same response in rats suggests that harm aversion is deeply conserved in the mammalian brain — meaning it may have developed early in the history of animal life on Earth. If that is the case, then it may be shared by many other animal species, too.
"The common misconception for decades has been that animals are aggressive by nature, and humans are special in that they overcome their animalistic drive, that compassion is what separates humans from other animals, that it is something divine," Inbal Ben-Ami Bartal, a postdoctoral researcher at the University of Chicago, tells Inverse. Bartal was not involved in the new study.
"But if you look closely at nature, you'll realize that many mammalian social species demonstrate elements of empathy, and pro-social behavior, helping, cooperation, care-taking, consolation, emotional support, social learning, etcetera," she says.
The results also suggest that the biology underlying harm aversion may be manipulated — enabling scientists to develop better treatments for brain conditions that affect empathy and the ability to recognize another's emotional and bodily state.
Share and share alike
In two experiments, neuroscientists tested rats to see if they would give up candy to avoid hurting their mates.
In the first experiment, the researchers put rats in a box with two levers, one of which dispensed a sugary substance. They let the rats choose which lever they preferred by letting them click the levers 24 times.
Once they had picked, the researchers repeated the experiment, but this time the lever that dispensed candy simultaneously delivered an electric shock to a neighboring rat. The rat pulling the lever could see and hear their neighboring rat, meaning they would hear them squeaking in pain due to the shock.
When the rats realized they hurt another rat by pressing the candy lever, they stopped.
It didn't matter whether the other rat was a known litter mate or not, the researchers report. This is in line with data from other experiments showing rats will free unfamiliar rats from traps, and that rats freeze when an unfamiliar rat near them gets a shock, the researchers say.
Intriguingly, the rats' personal experiences seemed to matter: Rats were more likely to stop pressing the lever if they themselves had previously been shocked.
Effort also plays a part, the researchers say: Rats were less likely to stop pressing the candy lever to avoid harming their neighbors if they were only given less, rather than no food reward.
“Our rats clearly preferred the option that only gives them food, showing that, like humans, they’re actually trying to avoid harm to other individuals,” Christian Keysers, neuroscientist at the Netherlands Institute for Neuroscience and lead researcher on this study, said in a statement provided to Inverse.
"It shows that the moral motivation that keeps us from harming our fellow humans is evolutionary old."
Empathy in the brain
In the second experiment, the researchers drilled down on the brain circuits thought to underlie this kind of empathetic behavior. In humans, an area of the brain called the anterior cingulate cortex appears to regulate these behaviors. In rats, previous work suggest that they share at least some of this neurobiology, with certain regions of the brain activated when they witness another rat's pain.
When the researchers used anesthesia to tamp down brain activity in this area, the rats were less likely to show the same altruistic behaviors in response to another rat in pain.
"That humans and rats use the same brain region to prevent harm to others is striking,” Valeria Gazzola, researcher at the Netherlands Institute for Neuroscience, said in a statement.
“It shows that the moral motivation that keeps us from harming our fellow humans is evolutionary old, deeply engrained in the biology of our brain and shared with other animals.”
In fact, the biology governing this trait may be some 100 million years old — the point in mammals' evolution when the ancestors of humans and rodents are thought to have diverged.
“It shows that the mechanism that humans have not to harm other individuals is not something new, and fragile, and recent,” Keysers said.
Jean Decety, professor at the University of Chicago, notes that the results suggest empathy and harm aversion are critical traits in many species' evolution.
“This is a great study that shows how animal models contribute to our understanding of basic physiological mechanisms that are involved in harm aversion and empathy,” Decety tells Inverse.
“Such mechanisms are highly conserved across species and therefore are similar in humans," he says.
Decety was not involved with this research.
Are rats altruistic?
While compelling, the results do not mean that rats are altruistic, or that they even care about other rats. But it does suggest they are averse to harming them.
The results add to a growing pile of evidence to suggest rats react to other rats' emotions, and specifically, their distress. But the new data is not enough to draw conclusions about whether they are truly altruistic in the same way that we think of humans to be. Bartal notes that applying a human worldview to animals is "prone to error."
"We still can't ask animals to report their subjective experiences, but we can learn quite a lot about the biological mechanisms that underlie these behaviors we can observe, which comprise part of the natural behavioral repertoire of many species," she says.
In this study, for example, the researchers have no way of knowing whether the rats stopped pressing the lever because they genuinely felt sorry for their neighbors, or because the squealing noise annoyed them.
But that line is easily blurred in humans, too, Keysers said.
“They may just avoid the option that harms the other rat because they don’t like that they have to listen to the other rat complaining about the pain,” Keysers said.
But whether the rats' motivation is truly altruistic or not, the fact that both rats and humans share this trait and the biology that underlies it holds promise for future research and drug development for conditions that affect empathy in humans, he said.
“Now that we have a model for harm aversion in rats, and we know the same brain regions are involved,” Keysers said.
“We can start to understand how that really works in the brain of the rat, to then ultimately help develop drugs in humans.”
"The more this line of research is accepted, the more converging evidence we'll get, and hopefully be able to construct the micro-circuitry involved in social behavior in the same way that has been done with other aspects of neural processing, like vision for instance."
Abstract: Empathy, the ability to share another individual’s emotional state and/or experience, has been suggested to be a source of prosocial motivation by attributing negative value to actions that harm others. The neural underpinnings and evolution of such harm aversion remain poorly understood.Here, we characterize an animal model of harm aversion in which a rat can choose between two levers providing equal amounts of food but one additionally delivering a footshock to a neighboring rat. We find that independently of sex and familiarity, rats reduce their usage of the preferred lever when it causes harm to a conspecific, displaying an individually varying degree of harm aversion. Prior experience with pain increases this effect. In additional experiments, we show that rats reduce the usage of theharm-inducing lever when it delivers twice, but not thrice, the number of pellets than the no-harm lever,setting boundaries on the magnitude of harm aversion. Finally, we show that pharmacological deactivation of the anterior cingulate cortex, a region we have shown to be essential for emotional contagion,reduces harm aversion while leaving behavioral flexibility unaffected. This model of harm aversion might help shed light onto the neural basis of psychiatric disorders characterized by reduced harm aversion,including psychopathy and conduct disorders with reduced empathy, and provides an assay for the development of pharmacological treatments of such disorders.