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.

65

13 comments, sorted by Click to highlight new comments since: Today at 11:20 PM
New Comment

It would not have occurred to me to test the veracity of "eating dirt" taken literally as a description of putting dirt in your mouth and swallowing it. I always took "eating dirt" as synecdoche for the general practice of "allowing kids to get dirty and suffer exposure to mostly-harmless microbes", and the references that you gathered above seem almost entirely to support that belief. Perhaps I'm not autistic enough to bother with the difference between "letting kids get dirty" and "letting kids literally put dirt in their mouth and swallow it"; perhaps this failure of hair-splitting caused me to have correct beliefs after all.

Me too! I never read anything specifically about "eating dirt", and when I started reading this post I assumed it meant "playing in the dirt". Now I'm confused. Is it really that common to hear someone recomending kids actually should eat dirt?

Edit: Nevermind, OP clarified in a comment.

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 think "don't let kids eat dirt" originally had a much lower prior than "parachutes prevent falling injuries", that was specifically overcome by the impression of evidence that doesn't exist. There are lots of things in dirt we know are dangerous- pesticides, car exhaust, lead, animal waste... Maybe the benefits of dirt outweigh that, maybe they don't, we don't know because no one has checked. I also expect us to notice that parachutes fail without rigorous evaluation, whereas the effects of marginal dirt will be harder to notice.

I will be sad if people walk away with the impression that HH/OF are wrong, or that geophagy has concrete evidence against it rather than merely an absence of evidence and some very concrete reasons to think city dirt in particular is dangerous. That's why I went out of my way to specify "no evidence" and "only applies to dirt, not hygiene/old friends hypothesis in general", even though it was more annoying to write and I imagine makes the piece less fun to read. I suppose I could have leaned into that trade-off harder, but I don't expect it to make any difference in practice. I'm curious what you think I could have done differently that would leave people with more accurate beliefs.

[One possibility is "don't publish at all", but I think "hey, this common wisdom has much less support than you think" is in general useful to say when true and that banning the genre would be net negative].

It sounds like the problem might actually be mainly the title? Instead

No one has actually checked whether (Literally) Eating Dirt Benefits Kids

or something.

A few people have commented "no one meant eat dirt literally", so let me address that. 

This research was commissioned in service of a company that intended to sell dirt-enriched vitamins (who I must say handled the result very graciously).  I don't know anyone who proactively supplies their kid dirt to eat (although I think they might have bought those vitamins), but I've seen multiple friends inhibit their urge to stop their kid from eating dirt by quietly chanting "hygiene hypothesis" under their breath. I have occasionally seen parents soothe themselves through daycare-induced illness with "good for their immune system", but not nearly as often or intently as they do with dirt. I personally thought the data for "don't freak out about your kids eating food off the ground" was stronger than this,  was surprised by the result, and expect parents I know to be surprised by it. 

I'm not sure I would call eating dirt a tenet of modern parenting. Most parents will stop their children if they see them eating dirt. It's more a question of how hard you try to stop them. 

this may be a bubble thing- the parents I know generally won't, and the stated reason will be that eating dirt benefits the children's immune system.

On the other hand, it will not stop most children from intentionally ingesting quite some amount of dirt.

I was also researching this when my kids were small, and I agree that this deserves more scrutiny. I was able to unearth at least a bit more. First: How much dirt do kids actually eat? 

the median estimates of soil ingestion from the eight tracers ranged from a low of 9 mg/day (Y) to a high of 96 mg/day (V); ... One child had soil ingestion values ranging from 5 to 8 g/day 
-- How much soil do young children ingest: An epidemiologic study (Science Direct) scanned PDF found here

From personal experience, these numbers look pretty low. My kids definitely ate more than that - at least temporarily. 

This is also the sentiment of this interesting and somewhat comprehensive article on the matter:

Eating Dirt (Gerald N. Callahan, US National Institutes of Health)

The risks of eating dirt focus on serious infections, which seem rare. So I looked at incidence rates of gastrointestinal infections and found

5% of the U.S. population has been infected with Toxocara. Globally, toxocariasis is found in many countries, and prevalence rates can reach as high as 40% or more in parts of the world
-- Toxocariasis (CDC)

and

The prevalence of any FGID [Functional Gastrointestinal Disorders] in infants age ... 13-48 months was ... 11.3%... The most common disorders were ... functional constipation (9.6%) in toddlers.
-- Prevalence of Functional Gastrointestinal Disorders in European Infants and Toddlers (PubMed)

Which are higher than I would have expected, but if you think about FGID as vaccination it makes more sense that many toddlers get it. Better earlier than later.

Interestingly, geophagy is common among primates.

My summary is the same from this piece on eating dirt in the German magazine Spiegel: 

CONCLUSION: Dirt does not clean the stomach, but the stomach cleans the dirt. So it doesn't matter if children swallow some sand and their behavior is no reason to restrict their visits to the playground. On the contrary: if toddlers come into contact with many different microbes, their allergy risk is reduced. But they shouldn't be encouraged to eat dirty food.

ADDED: Dirt from your own house and garden is most likely fine if

  • the house is not newly constructed, but nature had time to cultivate it for some years,
  • there are no sources of pesticides or pollutants in the vicinity (includes e.g., lead paint), and
  • no non-domesticated animals roam close to the house.

EDIT: Typos, clarity.

Isn't the fact that almost all babies/toddlers seem to have a desperate desire to stuff dirt in their mouth some evidence that (at least in the ancestral environment), this is good for them? Or at least not actively bad?

Add that to the HH, and it seems like something which doesn't need a huge amount of evidence.

I agree that if you're in a highly polluted area (i.e. everywhere urban or cultivated) this argument doesn't apply.

I am surprised this writeup didn't mention physical aspects of geophagy. I always thought that tooth-wear was the main hazard of eating dirt. There's plenty of research on geophagy and tooth-wear, both direct like kaolin, and indirect like stone-milled grains which has a big impact in dental specimens in archeology.

Kids of dirt eating age will change their teeth in a few years anyway, so I think tooth-wear is less concerning.

New to LessWrong?