Rats avoid to hurt other rats: Findings shed light on human empathy disorders

Summary: Rats show altruistic behavior and avoid harming other rats. Researchers report harm aversion is deeply engrained in our biology. The findings pave the way to increasing harm aversion in those with empathy disorders, such as psychopathy and sociopathy.

Source: KNAW

Most humans feel bad about hurting others. This so called ‘harm aversion’ is key to normal moral development and is reduced in violent antisocial individuals. Unfortunately, little is known about what makes us harm averse and we lack effective pharmacological treatments for violence in psychopathic criminals characterized by a lack of harm aversion. In a new paper published in the leading journal Current Biology, a team of neuroscientists of the Netherlands Institute for Neuroscience (NIN) shows that male and female rats show harm aversion. This phenomenon depends on the same brain region associated with empathy in humans. This indicates that harm aversion is deeply engrained in our biology, and paves the way to future work on how to increase harm aversion in psychiatric diseases.

Candy for me, pain for you

We tend to believe that only humans have moral sentiments. Animals are selfish. In the struggle for life, they should care about themselves and their children – not strangers. In this study the researchers investigated whether this is true. The scientists gave rats a choice between two levers they could press to receive candies (sucrose pellets). After the rats had developed a preference for one of the two levers, the scientists rewired the delivery system so that pressing the preferred lever also deliver an unpleasant electric stimulation to the floor of a neighboring rat. The shocked neighbor reacted by squeaking their protest. Rats stopped using their favorite lever as soon as obtaining the candy meant hurting their neighbor. This was true whether the neighbor was a rat they had shared their home cage with, or 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.

To explore whether there is similarity between harm aversion in rats and humans, the researchers went one step further. In humans, functional magnetic resonance imaging experiments show that the anterior cingulate cortex, a region between the two hemispheres of the brain, lights up when people empathize with the pain of a fellow human. The researchers had recently shown that the same region in the rat contains emotional mirror neurons – neurons that map the witnessed pain of another rat onto the witness’ own pain neurons. In the present study, they reduced brain activity in the same region in the rat by injecting a local anesthetic and observed that rats then stopped avoiding to harm another rat for candy. “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”, comments Dr. Valeria Gazzola, one of the senior authors of the study and group leader at the NIN.

This shows a rat
We tend to believe that only humans have moral sentiments. Animals are selfish. In the struggle for life, they should care about themselves and their children – not strangers. The image is in the public domain.

Are rats altruistic?

Does that mean that rats care about the welfare of other rats? This question is difficult to answer. “Perhaps a rat stops pressing the harmful lever because it doesn’t like to hear another rat squeak – just as we do not like to hear a crying baby on a transatlantic flight. Perhaps they do so because they really feel sorry for their neighbor. We don’t know whether our rats had a selfish or altruistic motivation. But I would argue that we don’t always know the motives behind the good deeds of humans either” adds Prof. Christian Keysers, group leader at the NIN. “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”.

About this neuroscience research article

Media Contacts:
Christian Keysers – KNAW
Image Source:
The image is in the public domain.

Original Research: Open access
“Harm to Others Acts as a Negative Reinforcer in Rats”. Julen Hernandez-Lallement, Augustine Triumph Attah, Efe Soyman, Cindy M. Pinhal, Valeria Gazzola, Christian Keysers.
Current Biology doi:10.1016/j.cub.2020.01.017.


Harm to Others Acts as a Negative Reinforcer in Rats

• Independently of sex and familiarity, rats avoid actions harming a conspecific
• Prior experience with footshocks increases harm aversion
• Rats show large individual variability in harm aversion
• Anterior cingulate cortex deactivation abolishes harm aversion

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 the harm-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.

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