A client came to me to investigate the effect of high altitude on child development and has given me permission to share the results. This post bears the usual marks of preliminary client work: I focused on the aspects of the question they cared about the most, not necessarily my favorite or the most important in general. The investigation stops when the client no longer wants to pay for more, not when I’ve achieved a particular level of certainty I’m satisfied with. Etc. In this particular case they were satisfied with the answer after only a few hours, and I did not pursue beyond that.

That out of the way: I investigated the impact of altitude on childhood outcomes, focusing on cognition. I ultimately focused mostly on effects visible at birth, because birth weight is such a hard to manipulate piece of data. What I found in < 3 hours of research is that altitude has an effect on birth weight that is very noticeable statistically, although the material impact is likely to be very small unless you are living in the Andes.

Children gestated at higher altitudes have lower birth weights

This seems to be generally supported by studies which are unusually rigorous for the field of fetal development. Even better, it’s supported in both South America (where higher altitudes correlate with lower income and lower density, and I suspect very different child-rearing practices) and Colorado (where the income relationship reverses and while I’m sure childhoods still differ somewhat, I suspect less so). The relationship also holds in Austria, which I know less about culturally but did produce the nicest graph.

This is a big deal because until you reach truly ridiculous numbers, higher birth weight is correlated with every good thing, although there’s reason to believe a loss due to high altitude is less bad than a loss caused by most other causes, which I’ll discuss later. 

[Also for any of you wondering if this is caused by a decrease in gestation time: good question, the answer appears to be no.]

Children raised at higher altitudes do worse on developmental tests 

There is a fair amount of data supporting this, and some even attempt to control for things like familiar wealth, prematurity, etc. I’m not convinced. The effects are modest, I expect families living at very high altitudes (typically rural) to be different in many ways from lower altitudes (typically urban) in ways that cause their children to score differently on tests without it making a meaningful impact on their life (and unlike birth weight, I didn’t find studies based in CO, where some trends reverse). Additionally, none of the studies looked specifically at children who were born at a lower altitude and moved, so some of the effects may be left over from the gestational effects discussed earlier. 

Hypoxia may not be your only problem

I went into this primed to believe reduced oxygen consumption was the problem. However, there’s additional evidence that UV radiation, which rises with altitude, may also be a concern. UV radiation is higher in some areas for other reasons, which indeed seems to correlate with reductions in cognition.

How much does this matter? (not much)

Based on a very cursory look at graphs on GIS (to be clear: I didn’t even check the papers, and their axes were shoddily labeled), 100 grams of birth weight corresponds to 0.2 IQ points for full term babies.

The studies consistently showed ~0.09 to 0.1 grams lower birth weight per meter of altitude. Studies showed this to be surprisingly linear; I’m skeptical and expect the reality to be more exponential or S shaped, but let’s use that rule of thumb for now. 0.1g/m means gestating in Denver rather than at sea level would shrink your baby by 170 grams (where 2500g-4500g is considered normal and healthy). If this was identical to other forms of fetal weight loss, which I don’t think it is, it would very roughly correspond to 0.35 IQ points lost. 

However, there’s reason to believe high-altitude fetal weight loss is less concerning than other forms. High altitude babies tend to have a higher brain mass percentage and are tall for their weight, suggesting they’ve prioritized growth amidst scarce resources rather than being straight out poisoned. So that small effect is even smaller than it first appears.

There was also evidence out of Austria that higher altitude increased risk of SIDS, but that disappeared when babies slept on their backs, which is standard practice now.

So gestating in Denver is definitely bad then? (No)

There are a billion things influencing gestation and childhood outcomes, and this is looking at exactly one of them, for not very long. If you are making a decision please look at all the relevant factors, and then factor in the streetlight effect that there may be harder to measure things pointing in the other direction. Do not overweight the last thing I happened to read.

In particular, Slime Mold Time Mold has some interesting data (which I haven’t verified but am hoping to at least ESC the series) that suggests higher altitudes within the US have fewer environmental contaminants, which you would expect to have all sorts of good effects.

Full notes available here.

Thanks to anonymous client for commissioning this research and Miranda Dixon-Luinenburg for copyediting.