Living with companion animals is immensely popular in the U.S. Over 60% of Americans live with a companion animal. And some live with more than one animal at a time. We sometimes speak about “dog people” or “cat people,” but we don’t often talk about “pet people” in general. Some academics are looking at how living with companion animals in general—often dogs, but sometimes cats, or both dogs and cats—differs from not living with them. Some studies have found that living with a companion animal can have both mental and physical health benefits for people. That being said, many criticize some studies that have looked into the links between companion animals and human health for methodological weakness. Not accounting for how pet “owners” and “non-owners” differ, and how this might affect study outcomes, are prominent examples of such methodological weakness.

In this study, researchers wanted to describe how people living with companion animals and those that don’t live with them differ. And they wanted to clarify how that difference “needs to be accounted for in observational research.” The researchers hope to show how the difference “are also associated with health outcomes, which may lead researchers to under- or over-estimate the impact of pet ownership on health in any observational studies.” Researchers rarely question if there is something different about people who live with companion animals which may also affect their health. To test this, researchers used survey response data from the 2003 California Health Interview Survey (CHIS). This is a very large and thorough state-level health survey. Their sample included responses from 42,044 adults “for whom Individual characteristics and self-reported cat and dog ownership were available.” Of those, 26.2% of people lived with a dog, 21.5% lived with a cat, and 8.5% lived with both.

Through statistical analysis with this huge sample group, the researchers found that people living with companion animals were more likely to be “single females or married, younger, White, live in more rural areas, live in homes, and belong to households where everyone is employed full time.” They found that people living with dogs were more likely to be home owners and have a higher household annual income than those not living with dogs. In terms of health, they found that people living with companion animals were more likely to have asthma. And dog owners were more likely to have higher BMIs. But, they noted no other differences in general health and BMI. The researchers emphasize that “the socio-demographic differences between pet and non-pet owners are not trivial.” And they further underline that “some of the health differences observed … could be over- or underestimated due to differences in socio-demographic variables such as age, race, gender, employment, income, and housing, and not necessarily” whether someone lives with a companion animal or not.

So, where does this leave companion animals and their advocates? It’s important to recognize, as the authors of the study do, that selection bias is “not a new problem.” It does not necessarily invalidate many years of observational behavioral research. The authors note that many methodologists are working to advance new methods to deal with such problems. In the meantime, while the main purpose of the study was to sort out methodological problems, the demographic information it highlights is valuable for companion animal advocates in and of itself.