I think part of the issue here is how one defines “placebo effect.” The casual usage refers to the effects of expectancy and belief, and clearly those won’t obtain where there isn’t any expectation or belief. However, it turns out that is only one element in a constellation of effects which are controlled for in clinical trials by having a placebo group, and many of these effects generate the impression of a specific treatment effect where the treatment is actually inert.
Some of these effects are functions of the disease, such as regression to the mean, spontaneous resolution, and other changes in symptoms that may follow a treatment without being causally related to it. Skeptics are just as likely as anyone else to interpret such changes as the result of the therapy, whether they are in the control group of a clinical trial or just trying out a treatment on their own.
Other effects that generate a false impression of therapeutic benefit, and which are often called “placebo effects” in the context of clinical trials, are functions of the conduct of the trial. The so-called Hawthorne Effect, for example, in which participants in a trial take better overall care of themselves and are more closely monitored while in the study, and consequently who improve even if the test therapy isn’t doing anything. Again, skeptics are as likely as anyone else to experience these effects.
And many of the cognitiv heuristics and other quirks that lead people to conclud a therapy is effective even when it isn’t (confirmation bias, availability heuristic, cognitive dissonance, and so on….) are just as likely to affect people who are dubiousl of faith-based beliefs as anyone else.
So while I think one can isolate the effects of belief and expectancy and hypothesize that self-identifying skeptics are less susceptible to these (though I haven’t seen any actual evidence to support that hypothesis), much of what fools us about the efficacy of medical therapies, and which is lumped under the general term “placebo effects,” seems no less likely to affect skeptics than others.
As for the literature in animals, there is unquestionably a strong placebo effect, both direct and via proxy, evident in clinical trials, so despite the almost certain lack of beliefs about their condition, animals appear to respond to inert therapies at much the same rate human patients do. Here are a few examples:
Caregiver Placebo effects in dogs with osteoarthritis
Placebo effects in canine epilepsy trials
Malek S, Sample SJ, Schwartz Z, Nemke B, Jacobson PB, Cozzi EM, Schaefer SL, Bleedorn JA, Holzman G, Muir P. Effect of analgesic therapy on clinical outcome measures in a randomized controlled trial using client-owned dogs with hip osteoarthritis.
. BMC Vet Res. 2012 Oct 4;8(1):185. [Epub ahead of print]
Is there a placebo effect in animals?