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A placebo can only work if someone (or something) believes it will. So how can animals be fooled by the placebo effect?

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If you know someone who swears that they’re living proof coconut oil is the cure for everything, you might be familiar with the placebo effect. That’s when something with no real pharmaceutical power, like a sugar pill, actually does help solely because someone believes it’ll work.

But weirdly enough, animals also seem to be fooled by placebos. Which seems like something that definitely shouldn’t be a thing, since they don’t know what medicine is. Scientists have a few ideas, though, as to why these effects happen.

And their very existence could have serious implications for scientific research, especially drug testing. We don’t know a ton about how the placebo effect actually works, even in us, but it’s thought that it largely hinges upon you believing what you’re taking has the power to fix what ails you. That’s why it’s kind of strange that dogs, rats, and other animals also seem to respond to placebos.

After all, a dog doesn’t know what a pill is, let alone that it’s supposed to fix something that’s broken. Still, the effect has been documented in multiple studies. For example, in a 2012 study, a group of 19 rats were trained to expose their faces to a painful heat source in order to get a treat.

Then they were injected with morphine, and offered the treat again. Since the morphine dulled the pain, they were less bothered by the heat. The eight that received saline instead of the painkiller were understandably less interested in the tasty reward.

After a couple rounds of this, the researchers switched to injecting both groups of rats with saline. Yet the originally morphine group were still mostly willing to brave the heat, even though they didn’t have the painkiller anymore. For them, the saline acted as a placebo.

And that’s probably due to conditioning: where an individual is trained to react in a certain way to a signal. It’s a phenomenon made famous by Russian physiologist Ivan Pavlov through his experiments with dogs, who were trained to associate the sound of a bell with a juicy steak. Eventually, the sound alone was enough to make them start drooling.

Conditioning can lead to a special kind of placebo effect, where the response to a medication is so strongly associated with the experience of receiving it that the body keeps responding even after it stops getting the actual drug, like seen in the rats in that 2012 study. They were conditioned to associate an injection with less pain, so even when the stuff in the injection changed, their pain was still dulled. But not all animal placebo effects can be explained by conditioning.

For example, a 2010 meta-analysis examined three studies where epilepsy medication was tested in dogs. The researchers found that 79% of the 28 pooches in the placebo groups had fewer seizures. It’s less likely conditioning was at play in these studies.

Still, scientists aren’t 100% convinced that a true placebo effect explains things, either. Something called the Hawthorne Effect could be the culprit instead, which is where someone, or an animal, improves just by being involved in a study. That’s because study subjects are closely monitored and cared for.

Basically, they get more attention, so they tend to do better. Or, it might really be a placebo effect, but in the people involved in the studies, not the dogs. The caregiver placebo effect can happen if the researchers or the animals’ owners expect the treatment to be working, which makes them more likely to report improvement.

And that’s something scientists have definitely seen. For example, a 2017 literature review re-analyzed five studies on cats with joint pain, and found that between about half and three quarters of cats on placebos were classified by their owners as improving. But using more objective measures, only between 10 to 63 percent actually improved, suggesting that some if not most of the supposed improvement was just in the owner’s heads.

And it’s actually a really big deal that these kinds of placebo or placebo-like effects can happen in animals, because we do a lot of research in animals. Before medications are prescribed by veterinarians, they undergo clinical tests similar to what we do with our drugs. And a lot of our pharmaceuticals are often tested in animals before they’re tested in people.

Placebo effect could make a dud seem like a wonder drug. Which can be especially dangerous because placebos usually just improve symptoms rather than treating the underlying cause. An animal with cancer might act less ill on a placebo, but their tumor could still be growing.

Or, placebos could make it seem like a good drug isn’t effective, if the control group on a placebo does overly well. Further studies of the placebo effect in animals and how it might mess with clinical tests could help researchers design studies that minimize these pitfalls. Because the phenomenon definitely exists, even if it seems like it shouldn’t.

And it just might explain why that friend of a friend of yours thinks that homeopathic drops are curing their pup’s arthritis. Thanks for watching this episode of SciShow! If you like learning about the brain and how it works, well, we have an entire channel dedicated to that!

You can head over to to check it out. [♪ OUTRO].