Most humans actively try to do all they can do to avoid harming other people. Psychologists refer to this behavioral trait as ‘harm aversion’ and it is considered a fundamental cornerstone of healthy moral development. Case in point, studies point to the fact that people with violent antisocial tendencies score low on this trait. That being said, not much has been known about how harm aversion works in the brain — until now, after researchers showed that rats also actively avoid harming their own kind.

Rodent empathy

Neuroscientists at the Netherlands Institute for Neuroscience (NIN) performed experiments with caged rats where they had the choice of pressing two levers, one of which delivered a tasty treat (sucrose pellets).

After this conditioning round, the entire setup was changed such that pressing the lever that offered candy also delivered an unpleasant electric shock to the floor of an adjacent cage where another rat was stationed. Pressing the lever caused the neighboring rat to squeak in protest.

Time after time, the rats stopped pressing the lever after they figured out that this would cause pain to the adjacent rat. This was true whether or not the neighbor had previously shared a cage with the lever-pressing rat or was a total stranger.

“Much like humans, rats thus actually find it aversive to cause harm to others” explains Dr. Julen Hernandez-Lallement, first author of the study and researcher at the NIN.

Previously, brain scans showed that a brain region nestled between the two hemispheres, called the anterior cingulate cortex, lights up with activity when people empathize with the pain of a fellow human.

This same region also becomes more active in rats that witness the pain of fellow rats. According to the researchers, the rats have emotional mirror neurons that trigger the animal’s own pain neurons when another rat is harmed.

When brain activity in the rats’ anterior cingulate cortex was reduced after a local anesthetic was administered, the affected rats stopped avoiding pressing the lever that harmed the neighboring rat.

“That humans and rats use the same brain region to prevent harm to others is striking. 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”, commented Dr. Valeria Gazzola, one of the senior authors of the study and group leader at the NIN.

This doesn’t necessarily mean that the study proves that rats have empathy and care about the welfare of other rats. Perhaps the rat stops pressing the lever because he simply can’t stand the squeaking noise, for instance. We can’t know for sure unless you ask the rodent and, unfortunately, that is not an option. But the same reasoning can be applied to humans also. How can we know for sure when a person acts out of altruism or selfishness?

The findings, which were published in the journal Current Biology, are extremely valuable nevertheless. More practically, they suggest that the anterior cingulate cortex can be targetted by pharmacological means to perhaps treat individuals who might engage in violent or psychopathic crimes.

“Whatever the motive, that we share a mechanism that prevents antisocial behavior with rats is extremely exciting to me. We can now use all the powerful tools of brain science to explore how to increase harm aversion in antisocial patients,” added Prof. Christian Keysers, group leader at the NIN.

This wasn’t the first time that researchers showed that rats have empathy traits. A study published all the way back in 1959 found rats refused to press a lever to obtain food when that lever also delivered a shock to a fellow rat (yes, just like this new study). Their protest was so strong that the rodents would rather starve than witness a rat suffering. Subsequent studies showed that rats refuse to follow a certain path in a maze if it resulted in a shock delivered to another rat or that rats who had been shocked themselves were less likely to allow other rats to be shocked.

In other words, rats seem to care about their fellow rat. The kind of care that seems to be in low supply among some humans.