Doctors have long observed that mock medical treatments, such as sugar pills and saline injections, make many sick people feel better. Scientists believe that these improvements can be partly explained by the power of expectations; truly believing that a pill will alleviate pain or nausea, for instance, may in fact make that discomfort subside. Though much remains mysterious about animal minds, it seems unlikely that pets are bringing these kinds of beliefs into the veterinary clinic. “I don’t think our patients have an idea about their disease that we can affect by saying, ‘Here’s a therapy that can make you feel better,’” McKenzie says. And yet a number of studies—of cats, dogs, and horses—have found that dummy drugs seem to help ailing animals get back on their furry feet.

A variety of mechanisms could explain these observations, some of which might also play a role in the human placebo response. One possibility is simple regression to the mean—the animals could be getting better independently of any medical intervention. Chronic diseases such as epilepsy tend to wax and wane, and pet owners might be more likely to enroll their dogs in a clinical trial or try new treatments when the seizures are particularly bad. In many of those cases, the seizures might be bound to improve on their own, simply as part of the natural course of the disease.

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Studies also show that people often change their behavior when they know they’re being observed. This so-called Hawthorne effect could help explain Munana’s findings. All the dogs in her study were on at least one anti-seizure medication in addition to levetiracetam, and pet owners might be more diligent about adhering to these drug regimens when their dogs are enrolled in a trial.

“Because they are in a study—and the owners know that their dogs are being scrutinized and the records are being scrutinized—maybe they’re more likely to give the medications on a more regular basis,” Munana says.

Animals in clinical trials might also receive better, more attentive veterinary care than they otherwise would. Some evidence suggests that gentle contact with humans may itself be therapeutic for certain creatures, including dogs and horses. And in some instances, classical conditioning could be at work. For example, rats that have regularly been getting insulin injections will still experience blood-sugar changes if they suddenly start receiving saline injections instead.

But in many cases, the most likely explanation is what’s known as the “caregiver placebo effect,” or “placebo effect by proxy.” In veterinary medicine, patients can’t speak for themselves. They can’t tell their doctors where they hurt—or even if they do. Instead, veterinarians have to rely on their own observations and judgments, as well as those of the animals’ owners, to infer how their patients are faring.