Even the researchers didn’t want to find what they found. Layla Parast, a Harvard-trained biostatician who does health analyses at the RAND corporation, approached the study with her colleagues fully expecting to see health benefits. The group “loves pets,” Parast insisted, as I probed for any evidence of anti-pet bias. She personally grew up with a dog and a cat, she claims.

“It was definitely our hypothesis that we’d find benefits. We assumed it would be a very straightforward thing to show.”

But no. The findings rather turned her understanding of the subject into a tangle of uncertainty. On Monday, the headline on RAND’s site was, “Largest-Ever Study of Pets and Kids’ Health Finds No Link; Findings Dispute Widely Held Beliefs About Positive Effects of Pet Ownership.”

Parast has already experienced social repercussions. “I've talked to a lot of friends of mine whose reaction was like yours and mine: No! This can't be true. What kind of ‘science’ are you doing?”

I imagine the genuine consternation on their faces matching that on mine. I’m not convinced that there is no link between pet ownership and health, and frankly neither is Parast. But it is worth looking at what kind of science she is doing, because it’s critical to how we understand health risks and benefits in much of life.

In this study, for example, the researchers looked at data from more than 5,000 households and analyzed children’s health by various measures, and then compared children in households without cats to children in those with cats. The kids with cats tended to have “better general health,” were less likely to have parents with concerns about their mood, behavior, or learning abilities, and were more likely to be physically active and deemed “obedient.”

This is all entirely consistent with what would be expected. What wasn’t expected was that once the researchers controlled for confounding variables, they had to report “no evidence for a beneficial effect of pet ownership for child health.” The pets didn’t make people healthier.

(Note that the researchers refer not simply to cats, but to “pets.” That is because the analysis studied the effects of owning cats and/or owning dogs. The findings were the same for both cats and dogs. Kids in dog-owning households tended to be healthier than those in pet-less households, but there was no evidence that dogs could account for that health difference. While I’m open to the possibility that this is true for cats, I’m not able to even comprehend it for dogs. So here I focus on cats.)

So if it’s not the pets themselves that are causing these health differences between pet-owning and non-pet-owning homes, what is?

Parast specializes in studying the sorts of health behaviors that aren’t amenable to randomized controlled trials. For example, smoking. It would be unethical to assign people to smoke cigarettes for 20 years and see if they developed emphysema. It would also be difficult to keep the test subjects blind. People tend to know when and what they are smoking.