Even though there is not a lot of biological evidence yet to prove that dogs have health benefits for humans, some companies have recognized that their employees are happier and more productive when they are with their dogs. Google, for example, states, “[Our] affection for our canine friends is an integral facet of our corporate culture. We like cats, but we’re a dog company, so as a general rule we feel cats visiting our offices would be fairly stressed out.” Amazon has a similar policy, simply requiring that employees register the dog and be responsible for good canine citizenship (barking and peeing are no-no’s). Other large companies with dog-friendly policies include Ben & Jerry’s Ice Cream, Clif Bar, the Humane Society headquarters, Build-A-Bear Workshop headquarters, and the software maker Autodesk. And, of course, many small businesses around the country.

If having dogs at work makes the humans less stressed, do the dogs feel happier too? The question is embedded in the much deeper riddle of animal emotions and gets to the heart of why we were doing the Dog Project.

For the most part, scientists have ignored the question of whether animals have emotions. This is peculiar because most pet owners are pretty sure that they do. Science, though, deals with things that you can measure, and, by definition, emotions are internal. Science has been able to measure only behaviors that are a result of an emotion. With humans, this is not a problem. You can always ask a person how he is feeling and deduce which emotion is associated with a behavior. The linking of subjective states and objective behaviors is an important step because different emotions may result in similar behaviors and expressions. For example, if you see someone crying, you might assume he or she is sad. But those could be tears of joy. The only way to know is to ask.

This inability to exactly determine emotions from behavior is why scientists have generally avoided the question of animal emotions. For example, a dog can’t tell you why he chews your slipper. But scientists have not always been so reluctant to venture into this. Charles Darwin devoted an entire book to the topic. In The Expression of the Emotions in Man and Animals, Darwin described how emotions like joy and fear have common manifestations in both animals and humans. Although Expression of the Emotions was his third book, after his famous books on evolution, it is the one that resonates most strongly today. The timelessness comes from his heavy reliance on dogs to illustrate his points. Richly illustrated with photographs and engravings, the modern reader can immediately identify with Darwin’s dogs.

Because humans and animals evolved from a common ancestor, Darwin deduced that we might also share basic emotional functions. If that were the case, animal emotions would help reveal the origins of human emotions. Unlike other scientists of his era, content to simply describe natural phenomena, Darwin wanted to understand why emotions manifested the way they did. Why, for example, does happiness trigger an upturned mouth as opposed to a downturned one?

Darwin formulated three principles of emotions that applied to man and animals. First, he said that emotions come from the brain. This was a pretty remarkable and correct intuition, considering that almost nothing was known about the brain in 1872. Second, emotional expressions build on natural movements. For instance, smiles are upturned because laughter triggers the closing of the eyes, and the contraction of the muscles around the eyes also raises the corners of the mouth. Third, Darwin believed that emotions manifest as the opposite actions of opposing habits. Darwin chose a dog to illustrate this principle, which he called antithesis.

When a dog approaches a stranger that appears hostile, the dog “walks upright and very stiffly; his head is slightly raised…; the tail is held erect and quite rigid; the hairs bristle… the pricked ears are directed forwards, and the eyes have a fixed stare.” These actions are defensive and may represent a prelude to an attack. The principle of antithesis states that the opposite emotion—joy—manifests with opposite motions. “Instead of walking upright, the body sinks downwards or even crouches…; his tail, instead of being held stiff and upright, is lowered and wagged from side to side.” The descriptions are as apt today as they were 150 years ago.

Darwin’s work on emotions was forgotten for more than a century. Although serious research in this area is beginning to attract scientists again, the vast majority still stays away from the knotty question of animal emotions. A major factor in scientists’ reluctance is that the study of animal emotions opens up an uncomfortable ethical question. If animals have emotions like humans, is it right to kill and eat them?

There have been a few exceptions. Within neuroscience, two people stand out. Kent Berridge, a psychobiologist at the University of Michigan, has extensively studied the link between reward systems in the brain and the expression of emotion in rats. And Jaak Panksepp, a neuroscientist at Washington State University and Bowling Green State University in Ohio, has been the strongest advocate for mapping animal emotions onto corresponding brain systems that are common to all mammals.

Reiterating what Darwin said, Panksepp has argued that only when we understand the emotional systems of our fellow creatures will we begin to understand the origins of human feelings. This is a compelling argument. When we look at the brains of animals, it is immediately apparent that there are many structures in common. The commonalities have traditionally been called primitive, reflecting scientists’ belief that they must have an old evolutionary origin. In the 1960s, the neuroscientist Paul MacLean used the evolutionary analogy to divide the brain into three parts: the reptilian brain (the basal ganglia), the paleomammalian brain (the limbic system), and the neomammalian brain (the neocortex). Although these divisions are overly simplistic, it is clear that only the neocortex is substantially different in humans and other mammals. The other two divisions—the basal ganglia and the limbic system—are largely the same from rats to humans. It is in these systems that Berridge and Panksepp believe that emotions originate.

The first difficulty in studying animal emotions lies in describing what an emotion is. Humans have a rich language for emotion, but even if you take something as basic and universal as love, you quickly realize the vast nuances that that word contains. There are so many different types of love that the word itself is inadequate. Assuming, for the moment, that our dogs love us, what kind of love would that be?

To proceed scientifically, we must set aside such subtleties. It helps to break emotion down to fundamental components: valence and arousal. Valence is simply goodness or badness, while arousal describes the level of excitement, ranging from calm to maximum excitement. Many human emotions can be plotted on a graph as a function of the combination of valence and arousal. Because the graph forms a circle, it is called the circumplex model of emotion. Positive emotions are plotted to the right, while negative ones are on the left. In the vertical direction, high-arousal emotions are at the top, while low-arousal ones are at the bottom.

Many psychologists have argued that the two-factor model is too simplistic. However, it provides an excellent starting point to understanding which parts of the brain give rise to the different emotions. As it turns out, the reptilian part of the brain, which we now call the basal ganglia, is closely associated with positive valence, while the limbic system is associated with arousal. By examining the relationship of activity in these different brain systems to the emotions experienced by human subjects, we can build an emotional brain map.