Humans and rodents have similar brain structures that regulate empathy, suggesting the behavior is deeply rooted in mammal evolution. Previous research has shown the much-maligned rodents assist comrades in need, as well as remember individual rats that have helped them-and return the favor. Now, a new study builds on this evidence of empathy, revealing that domestic rats will avoid harming other rats. In the study, published ... in the journal Current Biology, rats were trained to pull levers to get a tasty sugar pellet. If the lever delivered a mild shock to a neighbor, several of the rats stopped pulling that lever and switched to another. Harm aversion, as it's known, is a well-known human trait regulated by a part of the brain called the anterior cingulate cortex (ACC). Further experiments showed the ACC controls this behavior in rats, too. This is the first time scientists have found the ACC is necessary for harm aversion in a non-human species. The finding could have a real impact on people suffering from psychiatric disorders such as psychopathy and sociopathy, whose anterior cingulate cortexes are impaired. "We currently have no effective drugs to reduce violence in antisocial populations," [study co-author Christian] Keysers says, and figuring out how to increase such patients' aversion to hurting others could be a powerful tool. Whatever the motivation ... it's fascinating that the impulse to avoid hurting others is at least 93 million years old, which is when humans and rats diverged on the evolutionary tree.