Apart from some rear-guard behaviorists, few people hesitate to ascribe empathy to their dogs. But then dogs are mans best friend, freely credited with lots of human sentiments. For as much as we empathize with our canines, we have been stingy about recognizing empathy elsewhere in the animal kingdom, reserving it as a human trait. This belief is changing, however, as a growing line of research demonstrates not just empathys existence in other animals but its subtleties and exceptions as well. And they shed some interesting light on how we developed our capacity for caring for others.

Early Studies
The recent surge in empathy studies revives a line of research started almost half a century ago. In 1959 a paper by psychologist Russell Church in the Journal of Comparative & Physiological Psychology, provocatively entitled "Emotional Reactions of Rats to the Pain of Others." Church first trained rats to obtain food by pressing a lever. He found that if a rat pressing the lever saw another rat in a neighboring cage receive a shock from an electrified cage floor, the first rat would interrupt its activity--a remarkable result. Why shouldnt the rat continue to get food and simply ignore the other animals flinching? The bigger question was whether the rats that stopped pressing the lever were worried about their companions or just afraid that something bad might happen to them as well.

Churchs work inspired a brief flurry of research during the 1960s that investigated the presence of concepts such as "empathy," "sympathy" and "altruism" in animals. To avoid troublesome skepticism from colleagues, the investigators made sure to place the topics of their research in quotation marks; the prevailing behaviorist atmosphere made mention of animal emotions an anathema. Combined with the traditional emphasis on natures nasty side, this taboo ensured that these studies went largely ignored.

In the meantime, however, human empathy became a respectable study topic. First, in the 1970s, came studies of empathy in young children; then, in the 1980s, in adults. Finally, in the 1990s, researchers began placing humans in brain scanners to monitor them while they watched others who were in pain or distress or who had a disgusted facial expression--revealing many intriguing findings about activity in the brain. This field now produces new articles every week. But animal studies have lagged.

An Old Sorrow
This sluggish pace is changing. Slowly but steadily, nearly 50 years after Churchs rat study, the evolutionary origin of empathy is becoming a hot topic, reviving interest in studies of whether animals experience this complex and socially vital connection to others. Psychologist Stephanie D. Preston of the University of Michigan at Ann Arbor and I have argued that a basic neural process, first developed in our animal ancestors, underlies even the fancy kinds of empathy that only we humans are capable of. Seeing another person in a certain situation reactivates neural representations of when we ourselves have been in similar situations; this brain activity, in turn, generates a body state resembling that of the object of our attention. Thus, to see anothers pain may lead us to share it.

This empathetic capacity is in place on the very first day of a persons life. You can see it in any maternity ward, where all newborns will start crying as soon as one of them gets going. Artificial noise fails to cause the same reaction: babies are particularly sensitive to the distress calls of their own species. I have seen a similar spread of distress in young rhesus monkeys. Once, when an infant monkey had been bitten, it screamed so incessantly that it was soon surrounded by other infants. I counted eight of them climbing on top of the poor victim, pushing, pulling and shoving one another as well as the first infant. The response seemed automatic, as if the other infants were as distraught as the victim was and sought to comfort themselves as much as their companion.

A more rigorous and particularly revealing study of animal empathy came last year from psychology graduate student Dale J. Langford and her colleagues at McGill University in a paper entitled "Social Modulation of Pain as Evidence for Empathy in Mice," published in Science June 30, 2006. (Note that this time the word "empathy" is free of quotation marks; this absence reflects the growing consensus that emotional linkage between individuals probably has the same biological origin in humans and other animals.) This study was inspired by a puzzle that Langford and her laboratorys director, pain geneticist Jeffrey S. Mogil, found intriguing: when they tested mice from the same home cage in experiments that involved light shocks to the feet, the researchers noticed that the order in which the mice were tested seemed to affect their pain response. The first mouse would always show fewer signs of pain than the last. Was the last mouse being sensitized to pain by seeing others in pain? Or was something else at work?

To find out, Langford, Mogil and their colleagues devised an experiment in which pairs of mice were put through a so-called writhing test. In each trial, two mice were placed in two transparent Plexiglas tubes so that they could see each other. Either one or both mice were injected with diluted acetic acid, which is known to cause a mild stomachache. Mice respond to this discomfort with characteristic stretching movements. (This is less a "writhe," really, than a sort of discomfited restlessness.) The researchers found that an injected mouse would show more of this movement if its partner displayed the same behavior than it would if its partner had not been injected. Significantly, this increased display occurred only in mouse pairs who were cage mates.

Male (and not female) mice showed an additional interesting phenomenon when witnessing a strange male mouse in pain: its own pain sensitivity would actually drop. This counterempathetic reaction occurred only in male pairs that did not know each other, which are probably the pairs with the greatest degree of rivalry. Was the rivalry suppressing their reaction, or did they feel less empathy for a strange mouse?

(This gender effect reminds me of a wonderful study of human schadenfreude that Tania Singer, now at the University of Zurich, and her colleagues published in Nature in early 2006. The researchers found that in both men and women, seeing the pain of a person one has just cooperated with activates pain-related brain areas. But if a man felt he had been treated unfairly by another man in a previous exchange, his brains pleasure centers would light up at seeing the others pain. Such male antipathy toward rivals may be a mammalian universal.)

Finally, Langford and her colleagues also exposed pairs of mice to different sources of pain--the acetic acid as before and a radiant heat source that would cause pain if a mouse did not move away. Mice observing a cage mate suffering a stomachache withdrew more quickly from the heat source. In other words, the reactions of mice cannot be attributed to mere imitation, because a mouse seeing a companion in pain appears to be sensitized to any pain.

Foundation of Empathy
I admire this study greatly. It is not the kind of manipulation we would nowadays apply to primates, but it goes a long way toward confirming the tentative conclusions of the 1960s, with the benefits of more subjects and more rigorous controls. Although it does not prove that the mice feel vicarious emotions, it demonstrates that they experience a vicarious intensification of their own experience.

This demonstration justifies speaking of "empathy" outside of humanity--at least in some instances. Here we find an interesting division between psychologists, who tend to think in terms of top-down processes, and biologists, who tend to think from the bottom up. The top-down view considers the most advanced forms of empathy, such as putting yourself into another's "shoes" and imagining his or her situation, and wonders how this ability arises; the inevitable answer is advanced cognition, perhaps even language. Yet merely imagining someone elses situation is not empathy. Such imagination can be a cold affair, not unlike understanding how airplanes fly. Empathy requires emotional involvement.

Here the bottom-up view offers a better perspective. When we react to seeing someone display emotion and construct an advanced understanding of the other's situation, this process indeed involves--in humans and in some other large-brained animals--a great deal of cognition. But the emotional connection comes first; understanding and imagination follow. The mouse experiment suggests that the emotional component of this process is at least as old as our early mammalian ancestors and runs deep within us.