The observation that animals modulate immune response depending on perceptual cues is interesting, but you have to cross a fair inferential distance before you get anywhere near "the placebo effect".

This crossing goes something like, "you're manipulating something (lighting conditions) that has an effect only via the animal's perceptions", so that in effect you are "making the animal believe it's winter" - and then arguing that "since the result of this is a modulation of the immune system, it must be the case that beliefs can have an effect on the immune system" - and from there we bridge the final gap to "therefore this might be how the placebo effect works, by using the same mechanism whereby beliefs influence the immune system".

This final gap is speculative. Even if the hypothesis were true, it would only explain a select few of the control responses observed in experimental trials, not all of them. And the hypothesis doesn't mean that things like regression to the mean, measurement error, expectancy effects and so on are not also playing a role in control responses - it's only one more mechanism that joins these others in confounding the results of experimental trials.

More importantly, we don't know that what happens in placebo-controlled studies is anything like that, and the article adduces precisely zero evidence for that hypothesis. We have no idea whether humans respond to anything of the sort, and even if that was the case we wouldn't necessarily be able to harness the effect for curative purposes.

On top of all which Hanson adds an extra bit of speculation - "evolution shaped us to interpret being cared for in the same way that we would interpret other cues, such as long days, that we are in a situation of abundant resources" - still with not a shred of evidence.

All this reminds me of Mark Twain's quip: “There is something fascinating about science. One gets such a wholesale return of conjecture out of a trifling investment of fact."