As a child, Dr. Dhruv Kazi was obsessed with dogs. As a cardiologist and health economist, he wrote about their health benefits. But he didn't get one of his own until his early 40s. In 2019, he moved to Boston to take a job as the director of the Cardiac Critical Care Unit at Beth Israel Deaconess Medical Center. Then Covid hit. Living alone and working in the intensive care unit, Dr. Kazi said the first year of the pandemic was "immensely isolating." Everything changed in 2021 when he got Rumi, a high-energy, high-affection vizsla puppy. Thanks to Rumi, Dr. Kazi started spending more time outside, got to know his neighbors and had a much-needed dose of "positive energy" and "goofiness" injected into his life. "He was very much crucial to keeping my sanity," Dr. Kazi said.

Research dating back decades has found that people who own pets, especially dogs, tend to be healthier than people who don't. Studies show that having a pet is associated with lower blood pressure, a reduced risk of cardiovascular disease and lower rates of death after a heart attack or stroke. And a large review of studies published in 2019 found that owning a dog was associated with a 24 percent lower risk of dying from all causes over the course of 10 years. The benefit is so striking when it comes to heart health that the American Heart Association even has a scientific statement devoted to it, declaring that dog ownership "may be reasonable for reduction in cardiovascular disease risk." (The organization doesn't advise getting a dog for the sole purpose of heart health, though.)

"Pet owners in general, but dog owners in particular, have longer, healthier lives than people who don't have pets," Dr. Kazi said. "The correlation is very convincing. Now the question is: Is this relationship causal?" Some Pet Theories Experts think one potential explanation for the health benefits is that people who own dogs tend to be more physically active than those who don't. Dr. Adrian Bauman, an emeritus professor of public health at the University of Sydney in Australia, published a meta-analysis in 2012 that found most dog owners who walked their dogs met the standard exercise guidelines of 150 minutes of moderately intense physical activity per week. But only 60 percent of owners actually walked them.

"We have to distinguish dog ownership - having a dog in your household - from dog walking," Dr. Bauman said. In another study he conducted, there was no difference in mortality risk when dog owners and non-owners were equally active. (Dr. Bauman's dog, Jed, is a Cavapoo, a cross between a Cavalier King Charles spaniel and a poodle.) Other research backs up that having a dog doesn't guarantee you'll have healthy habits. And in fact, if your lifestyle is unhealthy, your dog's might be as well. Tove Fall, a professor of molecular epidemiology at Uppsala University in Sweden and a former veterinarian, has conducted research showing that if a dog has Type 2 diabetes, its owner is also more likely to develop it.

"You share your home environment with your dog," Dr. Fall said. "So if you are not living the healthiest lifestyle, maybe your dog is not doing that as well." (Dr. Fall has two dogs: Totte, a Labrador retriever mix, and Vega, a Dutch spaniel breed called a Kooikerhondje.) Experts say another way dogs may benefit people's health and longevity is by improving mental well-being. People who are single or live alone appear to gain most from the companionship a pet offers. Dog ownership "really has some substantial health benefits in countering the health consequences of loneliness, of isolation," Dr. Bauman said.

Lest cat lovers feel left out, a few studies have also found that owning a cat was associated with a lower risk of dying from a heart attack or stroke, possibly by providing stress relief. Or the health benefits of pet ownership may simply be an effect of demographics. Dog owners tend to be younger and richer than non-owners, characteristics that correspond with better health. In one large meta-analysis, when things like age, income and health behaviors such as smoking were factored into the statistical analyses, many health benefits of dog ownership disappeared. It's hard to tease apart whether dogs make people healthier or if healthier people are more likely to have dogs, Dr. Fall said. "If you're very frail and you cannot really take care of yourself, you're quite unlikely to get a puppy, right?"

The most compelling evidence for a causal relationship between dog ownership and health comes from a 2020 study that looked at the health of more than 336,000 Swedish adults who had suffered a heart attack or stroke. The researchers found that dog owners were significantly more likely to be alive a year after their heart attack or stroke than non-owners. The study was able to control for many confounding factors because Sweden has a national dog registry, and researchers could access detailed health records. "That's the best evidence we have that there is a causal effect," Dr. Fall said. "It's not just that healthy people get dogs."

The study also found that the benefit was strongest for people who lived alone. For them, owning a dog was associated with a 33 percent lower risk of death after a heart attack and a 27 percent lower risk after a stroke, compared with people who lived alone and didn't have a dog. "The dog is forcing you to get up and do things," Dr. Fall said. "You have to walk the dog, you have to feed the dog. And that gives you a purpose in life." Dr. Kazi said that while the evidence for a causal relationship is still not definitive, he's convinced that Rumi has improved his health. "I'm a believer," he said. "I think it's real."