system
Given the following post, respond as if you're commenting on Reddit:
human
Has there been any investigation of whether the placebo effect applies to animals?
gpt
Most certainly! Though the animal manifestation of the placebo effect may not be the same as you would imagine a human to exhibit the placebo effect. For example, if you give a human a placebo and tell them it is an antidepressant, they often begin to feel less depressed. This would not likely occur in an animal, as they have no expectation about a drug they have never taken before. However, if an animal is repeatedly given a certain drug, then is given a placebo dose of that drug, the animal will likely act in ways consistent with that drug's effects.

To look at a specific example of placebo in animals, let's consider researchers repeatedly administering morphine to rats. Morphine causes side effects such as vomiting and sleep. Rats who have been repeatedly given morphine have been observed to exhibit these symptoms when researchers are merely preparing to administer morphine. Similarly, rats who have been given morphine repeatedly can exhibit placebo analgesia, in which they have an increased pain threshold when placebo morphine is administered.

The mechanisms behind the placebo effect help to understand why this occurs for animals as well as humans. The placebo effect is driven by the anticipatory function of memory. Our brain is constantly trying to anticipate the present situation based on past events in order to guide our behavior. This process occurs automatically in order to conserve conscious resources, which are limited. A good example of this process is driving. When you get in a car, you automatically engage in many behaviors that enable you to skillfully drive, whereas when you were first learning to drive, it was effortful and cognitively demanding. Animals learn in the same way.

The same process happens with substance use. When you use a substance, your body learns to expect certain effects. For example, if you drink alcohol often, you may learn that you become more talkative and outgoing when drinking. Thus, if someone gives you O'Doule's and you think it's real beer, you might act outgoing and talkative, because that's the way you expect to feel. Animals also learn to expect certain effects from a drug, though perhaps not as explicitly as we do.

On an interesting and related note, this process is also responsible for tolerance. Your body learns to expect certain effects from a drug, and it compensates for those effects. If you had a high tolerance for alcohol, but someone injected you with alcohol without your knowledge, you would probably feel a lot more drunk than if you knew you were drinking alcohol, because your body wasn't informed that it had to prepare for being drunk.


Herrnstein, R., Placebo effect in the rat, Science, 138 (1962) 677–678.

Goldman, M. S., Darkes, J., Reich, R. R., & Brandon, K. O. (2006). From DNA to conscious thought: The influence of anticipatory processes on human alcohol consumption.

Kirsch, I., & Sapirstein, G. (1998). Listening to Prozac but hearing placebo: A meta-analysis of antidepressant medication. Prevention & Treatment, 1(2), 2a.

Nolan, T. A., Price, D. D., Caudle, R. M., Murphy, N. P., & Neubert, J. K. (2012). Placebo-induced analgesia in an operant pain model in rats. Pain, 153(10), 2009-2016.