The placebo effect is the phenomenon of reported or observed medical improvement in the absence of an active treatment. The placebo effect in humans has been found in several studies of pain and depression, and a recent study even suggested that when Parkinson’s patients thought they were receiving an expensive treatment (saline), they showed more improvement in motor function than when they received what they thought was a cheaper treatment.
Now it might make sense that humans would be influenced by thinking they are receiving a treatment. If you’ve felt better in the past after taking a medication, perhaps you were then conditioned to equate a pill with feeling better. In the future, even a pill that has no active ingredient could help you feel better. This type of effect was seen in dogs who learned that being put into a chamber was followed by a morphine injection (these happened to be Pavlov’s dogs). Soon, just being placed in the chamber led to a similar physiological response before they even received the injection.
Can animals experience a placebo effect without any previous conditioning? That is a little harder to know without objective measures, because animals can’t report to us directly about how they are feeling. However, perhaps an animal’s human caregiver has something to say about how their pet feels?
There is a variation on the placebo effect known as the caregiver placebo effect, where family members or clinicians rate that someone receiving a placebo has improved; interestingly, often the family member or doctor rates the amount of improvement as higher than the patient themselves does. In some cases, the ratings of improvement by the caretaker also influence the self-report of the patient – somehow the behavior of others, who believe you are receiving treatment (and therefore perhaps are less anxious), makes you feel better too! This is called placebo-by-proxy. This effect has been studied in humans, but could it happen in our pets too?