• Rats will stop doing something they like if it harms another rat, a study has found.
  • 'Harm aversion' is also found in humans, and is part of moral development.
  • The researchers traced the behaviour to an area of the brain known as the anterior cingulate cortex.

Rats, like humans, avoid actions that can cause pain to their fellow beings, scientists have found. This trait, known as harm aversion, is seen as an important part of moral development in humans but is reduced in violent antisocial individuals.

Researchers believe their findings, published in the journal Current Biology, could help scientists develop new drug treatments to increase harm aversion in patients who show psychopathic behaviour.

Professor Christian Keysers, study group leader at the Netherlands Institute for Neuroscience (NIN), said: “We share a mechanism that prevents antisocial behaviour with rats, which 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.”

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To investigate harm aversion in rats, the researchers gave them a choice between two levers they could press to receive sugary treats.

Once the animals developed a preference for one of the two levers, the scientists reconfigured the system so that pressing the favourite lever would also cause the rat in the next cage to receive an unpleasant shock while the treat was being dispensed. When the fellow rodents reacted by squeaking their protest, the rats stopped using their preferred lever.

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

The researchers then scanned the brains of rats and found a region of the brain, known as the anterior cingulate cortex, to become active. This same brain region has also been found to light up in people empathising with the pain of others.

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The team then reduced brain activity in the same brain region in the rodents by injecting a local anaesthetic and found the animals “stopped avoiding harming fellow rats for sweet treats”.

Dr Valeria Gazzola, one of the senior authors of the study and also group leader at the NIN, said: “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 ingrained in the biology of our brain and shared with other animals.”

Are babies born with a sense of right and wrong?

Early theorists in psychology mainly took the approach that babies are born without any sense of morality and have to learn it as they get older. We now know that although a fully developed sense of morality does not emerge until adolescence or later, babies already show signs of a rudimentary moral compass.

Consider a 2010 study by researchers at Yale University that involved babies as young as three months old watching a live ‘show’ of different shaped wooden blocks on a hill (the shapes corresponded to different characters, who either helped or hindered another character who was struggling to get up the hill).

The researchers found that the babies preferred looking at the helpful characters, suggesting early preference for altruistic social behaviour.

Similar research with five-month-olds has shown that they have a sense of ‘justified retribution’: they prefer characters who hinder a previously obstructive individual rather than help them.

A sense of fairness also emerges early. In a study last year by researchers at the University of Washington, 13-month-old babies watched a researcher who distributed crackers fairly or unfairly among two other adults.

When the infants were given a chance to interact with the researcher, they were more inclined to interact with a fair researcher than an unfair one, indicating that they had a preference for fairness.

Finally, a cute line of research has looked at babies’ inclination to respond to the needs of others, showing that already by age one they will offer comfort to a person who has hurt themselves, or try to help someone obtain an item that’s out of reach.

The spontaneity of these behaviours has led scientists to believe that a sense of right and wrong is not entirely learned, but rather indicative of an evolved predisposition towards moral goodness.

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Sara RigbyOnline staff writer, BBC Science Focus

Sara is the online staff writer at BBC Science Focus. She has an MPhys in mathematical physics and loves all things space, dinosaurs and dogs.