It’s Personhood Week here on Only Human. Today’s installment is about people and our fur babies. Monday’s post was about conception, Tuesday’s about the age of majority, and yesterday’s about identifying dead bodies. Tomorrow’s post, the last in the series, goes to neuroscientists who argue that “personhood” is a convenient, if illusory construction of the human brain.

I’d love to hear about how you guys define personhood, and why. Feel free to leave comments on these posts, or jump in to the #whatisaperson conversation on Twitter.

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I would be remiss, in a series about personhood, not to mention animal rights and the notion of non-human personhood. It’s incredibly interesting.* And yet… it’s not an issue that I can think about with much clarity or insight. When it comes to animals, my choices are full of contradictions and hypocrisies. I eat meat, wear leather, and endorse the use of animal models in medical research. On the other hand, I’m totally taken with the growing body of research demonstrating that non-human animals have cognitive skills once thought to be uniquely human. I believe animal cruelty is wrong and, as regular readers know all too well, I consider my dog part of the family.

So it’s that last thing I’m going to discuss here: pet-keeping. Nearly two-thirds of American families allow animals (animals!) to live with them. People are (arguably, more on that below) the only species to keep pets. Why do we bother? And what does our love of pets say about our personhood?

Scientists have proposed many different theories, as Harold Herzog outlines in the current issue of Animal Behavior and Cognition. Herzog, a professor of psychology at Western Carolina University in North Carolina, has been studying our relationship with animals for decades. His theory, which I find quite compelling, is that our love of pets comes from an innate predisposition to form emotional attachments, combined with rapid and powerful cultural evolution.

Herzog’s paper first addresses the question of whether humans are the only species to keep pets. You might assume we’re not, especially if you’ve seen the adorable “animal odd couple” YouTube clips like this one:

The thing is, Herzog argues, all of the non-human examples of inter-species attachments happen in households, zoos or wildlife parks. In other words, they happen when humans are around to facilitate. There’s one notable exception: A group of free-ranging capuchin monkeys in Brazil apparently adopted an infant marmoset named Fortunata. “The capuchins carried the marmoset around, played with it, and frequently fed the much smaller monkey,” Herzog writes. This illustrates that some non-human animals have the emotional capacity and care-taking skills to become attached to a member of another species.

Even if other species do keep pets, it’s pretty rare. Humans, in contrast, are downright pet-crazy. Why?

One set of theories says pet-keeping is an adaptive trait, meaning that it enhances our evolutionary fitness. Some studies have proposed, for example, that pet ownership leads to better health — everything from increasing the odds of surviving a heart attack to boosting mood and self-esteem. But this idea (no matter how much we pet owners would like to believe it) doesn’t have a lot of scientific support. In fact, some studies have shown that people who own pets have a higher risk of mental health problems, such as depression and panic attacks. What’s more, our pets can spread diseases through mites, tics, fleas, worms, and various viruses. And — this really floored me — every year more than 85,000 people get seriously injured after tripping over their pets.

In the same evolutionary vein, some researchers argue that pets improve our ability to attract mates or to care for our (human) children. There is a bit of evidence for the former. In a 2008 study, for example, a man approached random women and asked them out on a date. When the man took a dog along with him, his success rate increased three-fold. The idea that having a pet increases your ability to empathize with and take care of your children is also plausible but, according to Herzog, has not been formally tested.

Another set of pet-keeping theories says that the trait is an evolutionary byproduct. It could be, for example, that we love pets because we’re attracted to baby-like faces, or because of our strong parental urges, or because of our tendency to anthropomorphize animals.

Each of these ideas may have some merit, Herzog says, but he doesn’t believe any of them adequately explain the pet-keeping phenomenon, for several good reasons.

If pet-keeping were a purely (or even largely) biologically driven trait, it would be difficult to explain why its popularity has spiked in the last 200 years, and particularly since World War II — a tiny blip on the timeline of human evolution. As a rough marker of this change Herzog turns to Google Ngram, a tool that tracks the frequency of words published in books. If you put the word “pet” into Google Ngram, you’ll see a sharp rise since about 1960.

Similarly, if pet-keeping were biological you’d expect all human cultures to do it. While it’s true that most human cultures have pets in their home, the way they interact with them is remarkably variable. Herzog cites a study published in 2011 comparing pet-keeping practices in 60 societies around the world. The study found a large variety of species of pets, including some that seem quite odd from a Western perspective: ostriches, tortoises, bears, bats. The most common pet species is the dog, but even then, people are very different in the way they keep dogs.

Of the 60 cultures surveyed, 53 have dogs, but only 22 consider dogs to be pets. Even then, pet dogs are usually used for specific purposes such as hunting or herding. Just seven cultures regularly feed their dogs and let them live inside the house, and only three cultures play with dogs. The study’s general conclusion, as Herzog puts it: “The affection and resources lavished upon pets in the United States and Europe today is a cultural anomaly.”

Herzog’s own studies have measured cultural influences by tracking the popularity of dog breeds over time. He and his colleagues analyzed records from the American Kennel Club from 1927 and 2005. It turns out that, just like baby names, chart-topping songs, and other examples of popular culture, dog breed preferences follow a specific pattern, with most people choosing among a small number of breeds. The AKC recognizes 160 breeds, and yet nearly two-thirds of all registrations went to just 15 of them.

What’s more, breed popularity can shift rapidly due to cultural whims. After the Disney movie 101 Dalmatians was re-released, in 1985, Dalmatian registrations went up five-fold. And Old English Sheepdogs saw a 100-fold increase after the 1959 movie The Shaggy Dog. (Herzog notes, however, that not all popular dog references spur people to own them: The incredibly popular Taco Bell ad campaign had no effect on Chihuahua registrations.)

It’s hard for me to pinpoint my own motivations for having a dog. It’s a lot of extra work, not to mention money and time. Then again, I don’t really need an evolutionary explanation. All I can say is it makes me happy.

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*If you’re interested in learning more about the animal personhood movement/various philosophies of animal rights, I’d recommend:

—Charles Siebert’s story in The New York Times Magazine about Steven Wise, a lawyer who has filed lawsuits on behalf of several chimpanzees to contest their confinement in cages. Wise’s first case focused on a chimp named Tommy who is, as Siebert puts it, “the first nonhuman primate to ever sue a human captor in an attempt to gain his own freedom.”

—Virginia Morell’s National Geographic Q&A with Lori Marino, a scientist-advocate who studies the cognitive abilities of dolphins and other animals. “Person doesn’t mean human,” Marino says in the piece. “Human is the biological term that describes us as a species. Person, though, is about the kind of beings we are: sentient and conscious. That applies to most animals too. They are persons or should be legally.”

—A fascinating back-and-forth conversation between ethicist Peter Singer and Judge Richard Posner published in Slate in 2001.