Rainman – Link Between Precipitation and Autism

A new study published in the Archives of Pediatrics and Adolescent Medicine shows a positive correlation between counties in California, Oregon, and Washington with greater precipitation and a higher incidence of autism. While the results of this study are interesting, it needs to be put into proper context. Also of note, the authors had presented early results from this data previously.

Correlation is not Causation

This type of study is a correlational study, which means it asks whether or not there is a statistical correlation between two variables – in this case the rate of autism and the amount of precipitation.  This type of data is extremely useful to medical science, but it has known limitations, which can be summarized by the statement that correlation is not causation.

I often see this principle used to dismiss correlation data entirely, but that is not the correct approach. Correlation, rather, needs to be considered in the proper context. When A correlates with B there are various possible interpretations: the correlation is a statistical fluke (coincidence); A causes B, B causes A, or both A and B correlate with another variable C, and there can be a variety of causal relationships among the three (or more) variables which would cause A to track with B.

Therefore, finding a correlation is a way to generate several hypotheses which can then be tested by further observations or research. That, in my opinion, is the best way to view correlational data – as a beginning step to help generate hypothesis. But they should not be used to reach firm conclusions.

It should also be noted that further correlations can be used to test various causal theories, and if multiple correlations all triangulate to a single causal hypothesis that can lead to a fairly confident conclusion – even in the absence of other evidence. For example, smoking correlates with certain types of lung cancer. There are no prospective studies in humans to establish that smoking causes lung cancer, but we can be confident that it does because the correlation holds up no matter how you choose to look at it. If smoking causes cancer (as opposed to other causal hypotheses stemming from the correlation) then we predict that increased duration of smoking increases risk of lung cancer, that stopping smoking decreases risk, that smoking unfiltered is more risky than filtered, etc. Each of the predictions turns out to be true, supporting the smoking causes lung cancer hypothesis.

Rain and Autism

I was immediately interested in how the authors of this study came by the notion that rain might correlate with autism rates. In their older publication they mention that they thought of it as a way to test the hypothesis that TV watching may be an environmental trigger for autism, and that increased precipitation correlates with greater TV watching. In their most recent publication they explore various other causal hypotheses.

For example, they discuss that increased rain leads to more time spent indoors. While this may lead to increased TV watching, it also leads to an increase in all indoor activities. It also leads to increased exposure to the indoor environment, and decreased exposure to the outdoor environment. Therefore there may be some indoor trigger or outdoor protective element.

The rain itself may be involved. Increased rain may increase exposure to atmospheric toxins.

The authors also raise the possibility that increased precipitation correlates with decrease sun exposure, which could lead to relative Vitamin D deficiency, which is the actual trigger for autism. I actually find this the most plausible scenario. Relative vitamin D deficiency has been linked recently to increased risk of a variety of neurological disorders, and has lead to an increase in the recommended daily allowance of Vitamin D for children.

But what the study does not discuss are those causal hypotheses that no one has yet considered (obviously because no one has considered them). This is always the primary weakness of pure correlation – there are so many variables that it is nearly impossible to anticipate them all.

Here is an example from an unrelated field. The research on the possible protective effects of alcohol and heart disease has been largely epidemiological (i.e. correlation). Many studies showed that those who drink a little every day are healthier than those who do not drink at all. It was not considered for many years after this data started to come out, however, that the group that does not drink at all contains ex-drinkers, who are unhealthy as a consequence of their prior alcohol abuse. That may seem like an obvious point now, but it was missed for a long time.

It therefore seems possible, even likely, that there can be a complex causal relationship hiding in this apparent correlation between rain and autism.

We won’t know until further studies are done. The authors of this study concluded:

These results are consistent with the existence of an environmental trigger for autism among genetically vulnerable children that is positively associated with precipitation. Further studies focused on establishing whether such a trigger exists and identifying the specific trigger are warranted.

This is the exactly correct way to word this conclusion – the results are consistent with an environment trigger (but do not prove that one exists), and the results warrant further research (but cannot be used to form reliable conclusions). We can now base predictions on various causal hypotheses stemming from this study as a guide to future research. Something interesting may ultimately come out of this.


Autism remains a poorly understood neurological entity, although it is an area of active research and we have a great deal of information upon which to base our thinking about autism. It is clear that there is a huge genetic component to autism, but it is also probably not a strictly genetic disorder (or, more likely, set of related disorders). While there is no clearly established environmental factor, most such disorders result from an interplay of genetics and environment.

I suspect, however, that the environment of the womb is the dominant environmental factor. Recent research is finding that subtle signs of autism are detectable earlier and earlier in an infants life. The environment of the infant may play a role in expression, but I am not convinced that any post-natal factors trigger autism in children that would otherwise not display any features.  This latest study may be providing a clue (if the correlation itself holds up to replication) of a post-natal factor, but right now we are in the wild speculation phase of thinking about these results.

What is clear is that more research is warranted.

Posted in: Neuroscience/Mental Health

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16 thoughts on “Rainman – Link Between Precipitation and Autism

  1. MedsVsTherapy says:

    From my quick glance, a fair analysis. Another significant secular trend to include as a covariate / competing hypothesis is the probably true growth in the phenomenon of getting kids diagnosed with various disorders. This could have been, and still could be, partly captured by things such as: rise in number of psychiatric assessments overall; rise in psych hospitalizations for young children (available from nat hosp discharge survey, example pubmed 17306773); rise in number of mental health professionals (available from licensing boards); changes in health insurance coverage favoring mental health services which would include decreases in HMO market penetration; rise in providers of autism services; and, I would guess more importantly than individual income, rise in cost-per-square-foot of home, or median home price. Mdn home price would reflect increasing savviness and sophistication which might include the elan of getting your child to the doctor for his or her dx of ASD – oh – rise in coffee shops, cruiser bike shops, and hyphenated last names would also capture some of these secular social phenomena that could be comingled with the likelihood of seeking an eval.

    It looks like pop estimates are from U.S. census. Those will introduce error – which could be enough to kill a study that has wispy evidence at best. Pop estimate for 2006 is totally based on 200 census plus some extrapolation/projection based upon total recognized pop growth distributed across U.S. per 200 demographics (i.e., if there are a lot of younger people in Oregon, and the young-people demographic has been projected to be growing, Oregon will get this thrown into its estimate).

    A more clean eval would be to simply look at increasing rates for some time span geared to match the year 2000, thus no error added in pop projections to 2006.

  2. David Gorski says:

    Actually, it’s not entirely clear how they got a correlation:

    They did do a bit of curious statistical legerdemain to get their “correlation,” too:

    But worst of all, the investigators made no attempt at all to control for urbanicity, which, as I have pointed out before, is very important in studies of this type.

    Bottom line: I must respectfully disagree with my fearless SBM leader that Waldman et al supports the need for additional studies, at least as far as any link between precipitation and autism goes. That is not to say that it is not worthwhile to study environmental links to autism. It is to say that following the lead of this study would almost certainly result in a dead end. In any case, this particular article doesn’t provide even mildly convincing evidence that there is a correlation between precipitation levels and autism, because (1) it doesn’t consider a key and very important confounder; (2) it does a transformation of data that is not explained or justified; and (3) may well have cherry picked data.

  3. OMG… Vitamin D deficiency is one potential drawback of exclusive breastfeeding, too! Has anyone looked at whether there is any correlation between exclusively-breastfed babies and later development of autism?

    Not to jump to any freakout conclusions without backup data, but I didn’t get warned about the need for Vitamin D supplements for breastfeeders until well after my boy was eating solids. I was a Sears cult attachment parent, and thought Breast Is Best, “Supplements? We don’ need no steenking supplements,” and that the innate supremacy of Nature would triumph over all. Then my boy ended up iron deficient, and despite feeding him the vitamin drops, it was hard to get that hemoglobin up until we finally started supplementing with C as well.

    Nowadays I wonder if his early nutrition played any part in his Aspergers syndrome? I really really think the Vitamin D link needs to be looked at, and if it proves to be problematic for exclusive breastfeeders, then pediatricians and OBGYNs need to know it and warn their patients to give their babies supplements. And no, I’m NOT one of those folks who thinks autism is a disease to be cured or eradicated. What I would like to see are more closely-scrutinized guidelines for parents to know when they need to supplement their child’s diet for the best possible neurological outcome for that child, be it ever so neurodiverse. :)

  4. HCN says:

    I looked at the link to their earlier data, and noticed lots of smoothing and skipping of data.

    Why are the rain levels posted for Washington State posted for only above and below 27″? Why is Clallam County not marked for high autism when one of its towns, Forks, has over 100 inches of rain per year?

    What I see is a group of researchers with very little familiarity with this fairly mountainous region. Some seem to assume that both Portland, OR and Seattle, WA are on the Pacific coast. They are not.

    Seattle, WA is on Puget Sound (kind of like a fjord) and is separated from the Pacific Ocean by the Olympic Peninsula… which has a mountain range that cuts down the amount of rainfall. Which is why the peninsula has a temperate rain forest.

    Portland, OR is also not on the coast. It is a seaport via the Columbia and Willamette Rivers. Astoria is on the coast two counties over, and there is also a coastal range that acts as a rain shadow. The wind from the Columbia Gorge can be very significant… and Portland has slightly more rain than Seattle.

    There are also lots of historical stuff as to why there is development where it is, including the placement of railway lines, the location of farming communities in certain valleys, the availability of coal (which used to power San Francisco), the charging of tolls on settlers to float down the Columbia River, etc, etc.

    It is a gross misstatement to just blame it on the rain.

  5. wertys says:

    Parts of Northern Tasmania have over 4000mm of rain (150 inches plus) and there are no variations to the normative rates of autism that have been recorded..(Australian readers note there are no correlations with having two heads either..)

  6. wertys says:

    BTW HCN is there any truth to the speculation you wrote that last post and carefully analyzed the data just so you could use that last line ? Nice one !

  7. wertys says:

    BTW HCN, did you write that last post just so you could use that last line ? Nice work !

  8. David – I agree that the correlation itself is dubious, which is why I added the caveat “if the correlation itself holds up.” I didn’t do a statistical analysis of the study because I don’t have the background to do that, so I’m glad that others have. That’s the beauty of peer review. If nothing shakes out when it goes through the mill (which looks like it may be the case), then this lead will die.

  9. Mark Crislip says:

    In Portland there is almost nothing we cannot blame on the rain. Living here can seem like All Summer in a Day:

    I will credit the Oregon rains with a lot, but not autism.

  10. HCN says:

    Hey, it rains more in Portland than in Seattle! Plus you have those ice storms!

    Can you credit Oregon assessment criteria more than the rain? See:

    Also, really… what are the relative rates of autism between Bend, OR; Yakima, WA and another similar sized high desert city in California (I don’t know a name, but I assume there is one along the I-5 corridor between Yreka and Sacramento)? Lets try to get some of the variables consistent.

  11. HCN says:

    Psst… Dr. Crislip, the site from the Washington State School for the Blind did not work. I found it by putting “summer day” in the search box on their main page (, and got:
    (by the way, it is just across the river from Portland, OR… so it is in the same general area).

  12. MedsVsTherapy says:

    “Blame it on the rain, yeah, yeah…
    blame it on the stars ah- buh uh uve…!”

    Wait! Hold the presses! Milli Vanilli has given me the next autism theory: the stars! Let’s look at the astrological signs of these autistic children!

  13. David Gorski says:

    Another good analysis:

    I’m completely with Joseph now. The Waldman et al study is a pile of crap. How it passed peer review, I’ll never know.

  14. Yeah – nice analysis. It certainly seems like the correlation itself is bunk. Thanks for the link, Dave.

  15. weing says:

    FWIW, here is my pet theory about what could cause an increase in autism incidence and blame it on big pharma to boot. The evidence is pretty strong that autism spectrum disorder has a genetic basis and multiple gene mutations have been found as published in Science this past summer. There are also studies showing that women taking oral contraceptives prefer the smell of MHC similar men and presumably would tend to mate with them. Given the widespread use of birth control pills during the courtship stage, we end up with a large number of MHC similar couples. They are more likely to share these recessive gene mutations that are expressed in their children as autism.

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