Rats don't usually come out into daylight, especially not on a busy morning in New York City. But there it was, head awkwardly jutting out in front of its body, swinging from side to side. What injured the creature, I have no idea, but its hind legs could no longer support its weight. The rat dragged them like a kid drags a garbage bag that parents have asked be taken out–reluctantly. The muscles in the front legs rippled as they propelled the body forward along the sidewalk. The rodent was surprisingly quick considering the injury. But its aimlessness suggested distress.

Two girls, no more than 15 years old, spotted the wounded rat from about 10 feet away. They held each other close, squealing and giggling, inching toward the animal theatrically. Staring them down, I scowled. How could they not appreciate this creature’s suffering or be touched by its desperation? I looked on, saying nothing.

In The Last Child in the Woods, journalist Richard Louv talks about "nature deficit disorder," something we urbanites have picked up over the last hundred years or so. He says that city-dwellers have become so disconnected from nature that they cannot process the harsh realities of the natural world, like the sight of an injured animal. But if those young women were suffering from urban disconnection, then why didn’t I—a city slicker through and through—react that way as well? What made me respond with empathy instead of disgust?

Evolutionary theorists believe that many of our behaviors are adaptive in some way. "Empathy probably started out as a mechanism to improve maternal care," says Frans de Waal, a primatologist at Emory University and author of The Age of Empathy. "Mammalian mothers who were attentive to their young’s needs were more likely to rear successful offspring."

These offspring were, in turn, more likely to reproduce, so being able to sense another’s feelings was beneficial because it helped mammals to pass on their genes—the ultimate prize in the game of life. Mammalian males also show empathy, de Waal says, because “the mechanism spread from mother-offspring to other relations, including friends."

Although there is still a lot about empathy that scientists do not yet understand, theories abound. From a mechanistic standpoint, some researchers believe that a specific type of neuron—called a “mirror neuron”—might be a key to empathy. These neurons fire both when an individual, carries out an action and when that individual watches another perform the same action. If the theory holds true, mirror neurons might connect us to other living things.

"But apes have mirror neurons too and yet they only do very sporadic empathy, much less than us," says Jonathan Haidt, a social psychologist at New York University.  "So they are only a part of the story."

Some scientists argue that hormones provide the rest of the narrative. And if hormones are the story's main characters, then its hero may be oxytocin—a neurotransmitter that some scientists call the “love hormone.” Researchers have shown that oxytocin, made in the hypothalamus, is involved in human trust, bond formation, generosity and, of course, empathy. One 2010 study, demonstrated that men feel more empathy toward crying children and grieving adults after receiving an aerosol shot of oxytocin compared with a placebo. The science of oxytocin is still in its infancy, however, and although some of the research indicates that the hormone enhances trust and caring, in some cases, it might suppress it.

In addition to the physiological underpinnings of empathy, humans have to be able to imagine another’s situation in order to feel empathy. According to de Waal, this means adding a crucial cognitive layer on top of it all—the “thinking” part of the empathy reaction. This cognitive layer is the reason we held our breath during the Boston marathon bombing as we watched paramedics, runners and law enforcement personnel run to aid the debris-covered victims. It is why we teared up when we saw the bloodstained pavement once the chaos had been cleared away.

In such moments, the benefits of being able to perceive another person's emotions are readily apparent (how else could one comfort a person in need?). But exactly what we humans have to gain from perceiving the pain of non-human animals is less clear-cut.

Pat Shipman, an anthropology professor at Pennsylvania State University and author of The Animal Connection, believes that it all comes down to domestication. “The ability to ‘read’ another species’ emotions underlies all successful domestication of animals,” she says. “If you are going to take a wild animal into captivity, you have to have a tremendous understanding of what that animal needs.” Shipman thinks that those humans who were able to perceive and anticipate the needs of other animals were more successful in keeping them than those who weren't as aware. “Domestication turns us into much more efficient hunters and gatherers. We don’t have to evolve that specific strength, shape or size—we can borrow it from our animal partners instead,” Shipman explains. In short, humans who were more empathetic were also more prosperous. Viewed that way, the girls who squealed at the sight of the injured rat probably wouldn’t have made good hunter-gatherers.

But those girls were not just unsympathetic toward the rat. They were disgusted by it. Could disgust, like empathy, be adaptive?

According to Valerie Curtis, director of the Hygiene Center at the London School of Hygiene and Tropical Medicine, disgust is the voice in our heads that tells us to avoid things—foods and animals—that could harbor “disease-like bodily emanations.” Rats definitely fit that bill.  

The Center for Disease Control and Prevention lists 11 different types of diseases that rodents can transmit to humans, including hantavirus pulmonary syndrome, hemorrhagic fever with renal syndrome and bubonic plague. Breathing dust from places where rats dwell or drinking water in which rodents have defecated are the main modes by which humans contract these illnesses. Of course, bites from infected rats also pose a risk. Researchers think that these diseases explain why humans tend to find rats revolting, but not other rodent species such as hamsters and guinea pigs, which historically have carried less disease. Similarly, when we recoil at the sight of squirming maggots or gag while throwing out rotting food, we are protecting ourselves from the pathogens that they might carry.

These feelings of disgust are evolutionary messages telling us to get as far away as possible from the source of our discomfort. Researchers believe that many areas of the brain participate in the formation of these messages, but the anterior insulae—located deep within a fold of the brain known as the lateral sulcus—are among one of the most important, Curtis says. “They help us monitor our bodies’ interiors and notice nausea.”  

But with conflicting signals from empathy and disgust flooding our brains, how does one emotion prevail over the other? “We are full of conflicting desires, that is the nature of human beings,” Curtis observes.  “At any one time we have to weigh different motives and make a decision what to do based on circumstances, so people may simultaneously want to comfort a sick animal and recoil from its open wound.” What you choose to do, she says, “depends on the strength of your disgust and the strength of your desire to care.”

And when it comes to short-term survival, disgust is often the strongest feeling, Haidt says. We might prefer to think of ourselves as compassionate—a quality that aids long-term survival—but when we find ourselves in potentially life-threatening situations, our immediate desire to keep on living, often expressed through disgust, tends to win out.  

That's why "disgust is much more powerful close up," Haidt says. People might feel a lot of compassion for other creatures in the abstract, but if you show them a sickly animal and ask them to touch it, their empathy won't always translate into action.

On the sidewalk at 9 a.m., I somewhat foolishly expected a crowd to gather around the rat. But the bystanders who weren't rushing to work were rushing to grab coffee at the nearest food-cart. And besides, if people, including me, don’t stop for faltering homeless people on the street, why would they stop for an injured subway rat? It occurred to me that I should spare the creature the agony of a slow death by dehydration, or of a quick one by predation. But what would people think if they saw me killing a rat on Lafayette Street? Would I even be able to stomach it? Sometimes cultural norms supersede even our most primal instincts.