“What you see are these big themes in human nature: part biology, part culture, part voodoo magic that we don’t understand,” said Herzog.

“I used to think pet-keeping was a fundamental attribute of human nature — it evoked our parental instincts. The thing is, I no longer believe that. I think that culture trumps biology, because there are cultures that don’t even have a word for ‘pet.’”

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About two-thirds of U.S. households contain at least one pet, and we are pretty darn choosy about what type of critters do and do not share our homes. Americans lavished about $60 billion on their pets last year and spent another $7.8 billion to get rid of pests — some of which looked a lot like the pets.

“A pest is anything you don’t like,” said Cindy Mannes, spokeswoman for the National Pest Management Association. She said people spend more money to eradicate ants from their homes than for any other critter, yet “some people have ant farms in their houses. I think about a little kid who has iguanas in their bedroom. There are some people who probably couldn’t walk into the room if they knew they were there.”

The line between pet and pest differs depending on where you are on the globe.

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Dogs are considered pets in Western countries, unclean vermin in some Middle Eastern and Asian countries and tasty entrees in still other places.

Even within the United States, there are differences based on geography and ethnicity. Vermonters are most likely to have pets than residents of any other state. District residents are the least likely to own pets. And African Americans have lower rates of pet ownership than whites, Asians or Hispanics.

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James Serpell, director of the Center for the Interaction of Animals and Society at the University of Pennsylvania School of Veterinary Medicine, said that culture is a big driver in pet/pest choice, but he also emphasized that fads and fashion can’t explain it all.

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“You could almost say that people imprint on the animals they grew up with,” Serpell said. “If you grew up with dogs, you tend to be a lifelong dog person. You may even have a preference for particular breeds of dogs. … If we were all following the same fashion there would only be a handful of breeds, but we don’t. We go off in all sorts of different directions.”

And where Herzog sees wallet-size dogs as a fashion trend, Serpell also sees them as a practical choice: As people move into urban areas with smaller homes and less greenspace, they want pets that require less room to roam. And a single dog lover may keep a cat instead because cats generally require less care.

Serpell says we also can’t discount biology, particularly brain chemistry. He gave the example of oxytocin, the hormone that is released when mothers gaze at their babies.

“Eye contact from dogs appears to trigger the release of oxytocin, the bonding hormone, in the human brain,” he said. “It also releases oxytocin in the dog’s brain.

“Wolves don’t do that. It looks like we may, over time, have selected dogs for that type of behavior because it turns us on, so to speak. It tells us that we’re loved and cared for, and in turn it tells the dog it is loved and cared for.”

You just aren’t going to get that fuzzy feeling from the mole that’s digging up your yard or the squirrel that’s ransacking your bird feeder.

A 1989 California study found that pet choice correlated strongly to a person’s personality. Men who owned horses, for example, tended to be aggressive, while horse-owning women tended to be easygoing. Bird owners were expressive and outgoing. Snake owners were unconventional, not that anyone really needed a study to figure that out.

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Some pet choices, however, defy logic. That’s where culture comes in.

“Would you have a bulldog?” asks Herzog. Yes? “Then you’re going to send your veterinarian’s kids to college. … In 40 to 50 years, we took an athletic animal that was bred to work and turned it into this giant, slobbering heap of a thing with itchy skin, difficulty breathing, cardiac problems … and they tend to fart a lot.”

Yet the bulldog was the fourth most popular breed in the United States last year, according to the American Kennel Club. (The top three were Labrador retrievers, German shepherds and golden retrievers.)

Pet-keeping goes back at least 12,000 years and probably further, experts say.

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Society’s wealthiest people often started the trend, and it eventually spread to the middle and lower classes, according MIT history professor Harriet Ritvo in a paper on the emergence of pet-keeping. In the United States and Britain, she said, pets have become commonplace only in the past two centuries, a powerful argument for culture as a driving force.

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When fashion, peer pressure and trends come into play, it’s not hard to see how a pet here and now could be a pest in another place or time.

Cats, for instance, were cherished and sometimes worshiped in ancient Egypt, where they were thought to be elegant good-luck charms. Medieval European Christians, however, thought cats, particularly black ones, were demonic pests. By the 18th century, cats had become household royalty. Although British monarchs are usually known for their dogs — King Charles and his spaniels and Queen Elizabeth and her corgis — Queen Victoria doted on her cats and was a driving force in making cats cool again in 19th century England.

In other corners of the world this century, common pets include ducks, sloths, tapirs, ocelots, coatis, capybaras, caimans, ostriches, tortoises and bears. For people who cuddle capybaras and tame tapirs, perhaps Fido and Fluffy would be annoying pests.

Note: In case you’re wondering about the critter bona fides of the people who produced and are quoted in this story, here’s the tally of their current pets:

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— Author and blogger Hal Herzog: Tilly, a black cat

— NPMA spokeswoman Cindy Mannes: Dog Skyler and cats Riley and Fe

— Prof. Harriet Ritvo: Three Siamese cats, Manny, Tony and Ollie

— Prof. James Serpell: Atticus the dog, Henry the cat, Basil the bearded dragon and 11 aquariums of unnamed tropical fish

— Reporter Bonnie Berkowitz: Spike, a bearded dragon

— Illustrator Patterson Clark: A pond’s worth of mosquitofish and some well-fed composting worms

— Graphics editor Lazaro Gamio: Laika the rescue pit bull

— Lead Animalia blogger Karin Brulliard: Enzo the cat