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(CNN) --
As we celebrate "National Pet Day" in the United States -- which falls on April 11 -- we have some surprising facts about the benefits of having pets at home.
We understand that every day at home is "love your pet" day and that you cuddle up to your kitties as their furry breasts gently rumble, a proven antidote to the stress of the day.
For me and millions of others, having a pet brings a circle of love into our lives: they give affection, we return it, and we're all better for it.
And science agrees on that.
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"I have a list of 10 health benefits [that] studies have shown pet owners have," said psychologist Harold Herzog, a pet-loving professor at Western Carolina University who has long studied the connection between humans and animals.
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"Higher survival rates, fewer heart attacks, less loneliness, better blood pressure, better psychological well-being, lower rates of depression and stress levels, fewer doctor visits, higher self-esteem, better sleep, and more physical activity," are just a few of the registered benefits of pet ownership, Herzog said.
But here's a surprise...
Herzog also points to studies that found pet owners "are more likely to be lonely, depressed and have panic attacks, more likely to have asthma, obesity, high blood pressure, gastric ulcers, migraine headaches, and use more medicine, etc.
What's going on?
As is often the case in science, studies have had mixed results.
Some research shows the benefits of owning a pet, other studies say there is no difference between the health of those who do and do not own pets.
Still more research suggests there might even be downsides to pet ownership (and we don't just mean yard poop).
That's how it is.
Despite the fact that we are convinced of the blessings that our fur babies bring to our lives, science has yet to definitively prove that pets are good for our health.
"A lot of us who have pets think, 'Oh, they must be uniformly fine for us,'" said Megan Mueller, co-director of the Tufts Institute for Human-Animal Interaction and proud owner of a dog and a guinea pig.
"We're finding that it's a little more complicated than we originally thought," he added. "I always say it's not a great question: 'Are our pets good for us?'"
"Who are pets good for, under what circumstances, and is it the perfect match between person and pet?"
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anxiety and mood
Pet owners certainly believe that their pets provide emotional support, especially during times of stress, Mueller said, and thankfully the science seems to back that up.
"There is some research showing that having a pet with you during an anxious event might help reduce the stress of that event," he said.
"Studies have repeatedly shown that people's good moods are increased and bad moods are decreased around pets," Herzog said.
"And we know there are immediate short-term physiological and psychological benefits to interacting with pets. I have no doubts about that."
But the same cannot yet be said about depression.
Herzog conducted 30 studies on the topic: Eighteen showed no difference in depression rates between people with pets and those without;
five concluded that having a pet relieved depressive symptoms;
five pets were found to worsen depression;
and the rest were inconclusive.
A study of the elderly by Mueller found that pet owners were about twice as likely to have had depression in the past, but reported no depression in the past week.
Did they get a pet and then get depressed, or did the pet help end the depression?
It is not clear.
Therapy and emotional support
One of the reasons the science is so iffy on the subject of pets and our health is that it's nearly impossible to conduct the "gold standard" of studies: a randomized controlled trial where the investigator controls for all factors and then randomly assigns a pet to the test group.
"It's really hard to do randomized studies because most people want to choose whether or not they want a pet and choose who their pets are," Mueller said.
Backed by a $9 million partnership between the Child Behavior and Development Branch of the US National Institutes of Health and the UK's Waltham Institute of Pet Grooming Sciences, some researchers have begun designing better studies to determine whether it is the animals that have the impact.
A 2015 study found that children with Attention Deficit Hyperactivity Disorder (ADHD) who read to real animals showed more improvements in sharing, cooperation, volunteering, and behavior problems than children with ADHD who read to them to a stuffed animal.
Another study found that autistic children were calmer and more interactive in the presence of guinea pigs than toys.
A four-month randomized study conducted at Vanderbilt University in Nashville gave children access to therapy dogs just before undergoing cancer treatment.
All the children enjoyed it, but there was no decrease in anxiety levels between the children in the test group and those in the control group.
Parents of children who had therapy dogs, however, showed a significant decrease in parental anxiety about pain and their children's ability to cope.
Dogs and cardiovascular health
An analysis last year of nearly 4 million people in the United States, Canada, Scandinavia, New Zealand, Australia and the United Kingdom found that owning a dog was associated with a 24% reduction in deaths from any cause.
If the person had already suffered a heart attack or stroke, having a dog was even more beneficial;
they were 31% less likely to die from cardiovascular disease.
The study has been criticized for not controlling for other diseases, social economic status and other factors that could confound the results.
However, another large study published at the same time found that people who owned dogs had better health outcomes after suffering a major cardiovascular event like a heart attack or stroke.
The benefit was greatest for dog owners who lived alone.
Heart attack survivors who lived alone and had dogs had a 33% lower risk of death compared to survivors who didn't have a dog.
Stroke survivors who lived alone with a dog had a 27% lower risk of death.
Of course, these cardiovascular benefits are only for dogs, not cats, horses, and the like.
Many suggest that potential exposure to exercise explains the benefit: The American Heart Association points to studies that found pet owners who walk their dogs get up to 30 minutes more exercise a day than those who don't.
But in an earlier interview with CNN, Dr. Martha Gulati, who is editor-in-chief of CardioSmart.org, the American College of Cardiology's patient education platform, said they still didn't know why.
"Is it the dog or is it the behaviors?" Gulati wondered.
"Is it because you're exercising or because there's a difference in the type of person who would choose to have a dog versus someone who wouldn't? Are they healthier or wealthier? We don't know those things."
Still, while "nonrandomized studies can't 'prove' that adopting or owning a dog directly leads to reduced mortality, these strong findings at least suggest this," Dr. Glenn Levine told CNN. chairman of the writing group for the American Heart Association study on pet ownership, in a previous interview.
However, the American Heart Association also says that pet ownership is a commitment of care that comes with certain costs and financial responsibilities, so "the primary goal of adopting, rescuing, or purchasing a pet" should not be to reduce risk cardiovascular.
Pets as 'personalized medicine'?
Another research method being used, Mueller said, is longitudinal studies, in which large numbers of people are followed over long periods of time.
The hope is that these studies, and more scientifically designed experiments, will uncover more precise reasons why a particular pet may or may not be a good fit for a person and their needs.
One day, Mueller says, it might be possible to 'prescribe' a dog for a young active child, troubled teenager or cardiovascular patient and know, however much science may know, what the likely outcome is for their health.
Perhaps we finally have data to put behind the "cat vs. dog" debate, or simply how and why a bird, fish, lizard, or gerbil might ease our stress and provide companionship.
Until then, fellow pet lovers, I intend to get back to what I intuitively know: my pets are some of the most loving "people" in my life, and that, if nothing else, makes them good for me.
This article was originally published in 2020.
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