Sometimes people imply that epistemic spot checks are a waste of time, that it’s too easy to create false beliefs with statements that are literally true but fundamentally misleading. And sometimes they’re right.

On the other hand, sometimes you spend 4 hours and discover a tenet of modern parenting is based on absolutely nothing.

Sorry, did I say 4 hours? It was more like 90 minutes, but I spent another 2.5 hours checking my work just in case. It was unnecessary.

Intro

You are probably familiar with the notion that eating dirt is good for children’s immune systems, and you probably call that Hygiene Hypothesis, although that’s technically incorrect. 

Hygiene Hypothesis can refer to a few different things:

1. A very specific hypothesis about the balance between specific kinds of immune cells.
2. A broader hypothesis that exposure to nominally harmful germs provides the immune system training and challenge that ultimately reduces allergies.
   1. One particular form of this involves exposure to macroparasites, but that seems to have fallen out of favor.
3. The hypothesis that exposure to things usually considered dirty helps populate a helpful microbiome (most often gut, but plausibly also skin, and occasionally eyeball), and that reduces allergies. This is more properly known as the Old Friends hypothesis, but everyone I know combines them.
4. Pushback on the idea that everything children touch should be super sanitized
5. The idea that eating dirt in particular is beneficial for children for vague allergy-related reasons.

I went into this research project very sold on the Hygiene Hypothesis (broad sense), and figured this would be a quick due diligence to demonstrate it and get some numbers. And it’s true, the backing for Hygiene and Old Friends Hypothesis seems reasonably good, although I didn’t dig into it because even if they’re true, the whole eating dirt thing doesn’t follow automatically. When I dug into that, what I found was spurious at best, and what gains there were had better explanations than dirt consumption.

This post is not exhaustive. Proving a negative is very tiring, and I felt like I did my due diligence checking the major books and articles making the claim, none of which had a leg to stand on. Counterevidence is welcome. 

Evidence

Being born via c-section instead of vaginally impoverishes a newborn’s microbiome, and applying vaginal fluid post-birth mitigates that

This has reasonable pilot studies supporting it, to the point I mentioned it to a pregnant friend.

There are reports that a mother’s previous c-sections lower a newborn’s risks even further, but I suspect that’s caused by the fact below

Having older siblings reduces allergies

Study. The explanation given is a more germ-rich environment, although that’s not proven.

Daycare reduces later allergies, with a stronger effect the earlier you enter, unless you have older siblings in which case it doesn’t matter

Study. Again, there are other explanations, but contagious diseases sure look promising.

Living with animals when very young reduces allergies

This one is a little more contentious and I didn’t focus on it.  When the animal appears seems to matter a lot.

One very popular study used to bolster Dirt Eating is a comparison of Amish and Hutterite children. Amish children get ~⅙ of the allergies Hutterite children do, which pop articles are quick to attribute to dirt “because Amish children work on farms and Hutterite children don’t.” But there are a lot of differences between the populations: dust in Amish homes have 6x the bacterial toxins of Hutterite homes, the children have much more exposure to animals, and drink unpasteurized milk. 

Limitations of Farm Studies

Even if Amish children did eat more dirt and that was why they were healthier, there’s no transfer from that to urban parks treated with pesticides and highway exhaust. They might be net positive, the contaminants might not matter that much, your park in particular might be fine, no one has proven this dirt is harmful, etc. But you should not rest your decision on the belief that that dirt has been proven beneficial, because no one has looked.

Mouse Studies

There are several very small mouse studies showing mice had fewer allergies when exposed to Amish dirt, but:

1. They are very small.
2. They are in mice.
3. The studies I found never involve feeding the mice dirt. Instead, they place it in bedding, or directly their nasal passages, or gently waft it into the cage with a fan. 

So eating dirt is bad then?

I don’t know! It could easily be fine or even beneficial, depending on the dirt (but I suspect the source of dirt matters a lot). It could be good on the margin for some children and bad for others. Also, avoiding a constant battle to keep your toddler from doing something they extraordinarily want to do is its own reward. What I am asserting is merely that anyone who confidently tells you eating arbitrary dirt is definitely good is wrong, because we haven’t done the experiments to check.

I think any of [communicable diseases, animals, unpasteurized milk] have more support as anti-allergy interventions than dirt, but I hesitate to recommend them given that a high childhood disease load is already known to have significant downsides and the other two are not without risks either.

Epilogue

The frightening thing about this for me is how this became common knowledge even, perhaps especially, among my highly intelligent, relatively authority-skeptical friends, despite falling apart the moment anyone applied any scrutiny. I already thought the state of medical knowledge and the popular translation of that knowledge was poor, but somehow it still found a way to disappoint me.

My full notes are available in Roam.

This post was commissioned by Sid Sijbrandij. It was preregistered on Twitter. I am releasing it under the Creative Commons Attribution 4.0 license. Our initial agreement was that I would be paid before starting work to avoid the appearance of influence; in practice I had the time free and the paperwork was taking forever so I did the research right away and sat on the results for a week.

Thanks to Miranda Dixon-Luinenburg⁩ for copyediting.