Right. Or from another angle: people who do not have dogs are considered pariahs, so the dogless are getting a nocebo all the time. So when they take the placebo (dog) their increase in well being would mostly be through the elimination of the nocebo effect.

I see. Well, a placebo (belief-based) effect is part of the whole psychological effect (including interaction). It would probably even interact with the interaction - e.g. a belief that dogs are good for you would probably encourage the kind of interaction with the dog that makes the dog good for you, in which case the belief is an integral part of the overall mechanism.

So this is really a specific case of trying to tease apart different components of the overall psychological mechanism. In this case I don't think there's anything special about the "placebo" component of the overall mechanism that we especially need to tease apart from the other components. Sure, the placebo component is contingent on the belief that dogs are good for you and at some point in the future the tribe may lose that belief, and that's something to worry about. But the placebo component may not be the only component that is contingent on something that the tribe may lose over time. For example, maybe the beneficial psychological effect of dogs is contingent on the lifestyle of the tribe, which can affect how the tribe interacts with the dogs.

Still, a scientist might want to tease apart all the psychological effects experimentally. Probably not doable. There's only so much you can discover without opening the black box (e.g. conducting experiments on the tribe that are unethical).