This rubs me wrong for the same reason that “no evidence for...” claims rub me wrong.
We have a probably-correct model, the hygiene hypothesis broadly understood. We have a plausible corollary of that model, which is that kids eating dirt helps their immune system (I had never heard this particular claim before, but since you mention it, it seems like a plausible corollary). We should have a low-but-not-ridiculously-low prior on this.
(probably some people would say a high prior, since it follows naturally from a probably-true thing, but I don’t trust any multi-step chain of reasoning in medicine)
When I read the title, I thought “Oh! I guess someone showed the specific behavior of eating dirt doesn’t help, so I should update against the hygiene hypothesis!” But the post presents no evidence this is wrong. It’s just saying there are no studies of it.
This seems kind of like framing the proverbial parachute point as “‘Parachutes prevent falling injuries’” Is Basically Made Up”. It’s not made up! It was assigned a high prior based on other things we know! Nobody has given us any evidence for or against that prior, so we should stick to it.
I’ve tried to address your point about psychiatry in particular at https://slatestarcodex.com/2019/12/04/symptom-condition-cause/
For the whale point, am I fairly interpreting your argument as saying that mammals are more similar, and more fundamentally similar, to each other, than swimmy-things? If so, consider a thought experiment. Swimmy-things are like each other because of convergent evolution. Presumably millions of years ago, the day after the separation of the whale and land-mammal lineages, proto-whales and proto-landmammals were extremely similar, and proto-whales and proto-fish were extremely dissimilar. Let’s say in 99% of ways, whales were more like landmammals, and in 1% of ways, they were more like fish. Some convergent evolution takes place, we get to the present, and you’re claiming that modern whales are still more like landmammals than fish—I have no interest in disputing that claim, let’s say they’re more like landmammals in 85% of ways, and fish in 15% of ways. Now fast-forward into the future, after a billion more years of convergent evolution, and imagine that whales have evolved to their new niche so well that they are more like fish in 99% of ways, and more like mammals in only 1% of ways. Are you still going to insist that blood is thicker than water and we need to judge them by their phylogenetic group, even though this gives almost no useful information and it’s almost always better to judge them by their environmental affinities?
(I don’t think this is an absurd hypothetical—I think “crabs” are in this situation right now)
And if not, at some point in the future, do they go from being obviously-mammals-you-are-not-allowed-to-argue-this to obviously-fish-you-are-not-allowed-to-argue-this in the space of a single day? Or might there be a very long period when they are more like mammals in some way, more like fish in others, and you’re allowed to categorize them however you want based on which is more useful for you? If the latter, what makes you think we’re not in that period right now?