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Autism and vaccines: correlation is not causation

by Donald Prothero, Jan 22 2014

Just after the beginning of the year, leading anti-vaxxer and former Playboy Playmate Jenny McCarthy was in the news again. Several years ago, she apparently told Time magazine that her son Evan didn’t have autism, but (as doctors have long suspected) Landau-Kleffner syndrome. Then in January, McCarthy made several angry denials of this old interview. Frankly, I doubt that a source like Time magazine misquoted her—I think she’s lying again. She now claims that Time magazine inaccurately reported the facts! I think my irony meter just broke. Jenny McCarthy was a washed-up actress with nothing but a series of low-brow movies and TV shows to her credit until she became the national spokeswoman for the anti-vaxxer movement—a movement which was,  in a form of supreme irony, legendary for an inaccurate grasp of the facts, then shifting the goal posts on their demands when the mercury was removed from the vaccines and there was still no effect on rates of autism. She rejects mainstream science, yet cites biomedical science in support of her claim that her son really has autism. I think we should reject her claim outright, because she (like many anti-vaxxers) have no clue what mainstream science and medicine are about.

Now she and her anti-vaxx cohort are demanding that medicine and drug companies should “green the vaccine”! What the heck do they think the FDA is for? All vaccines must pass rigorous testing and quality control through the FDA, which has one of the most stringent standards for drugs and medicine in the world. Fat chance that she and her fellow science-denying protesters are going to help the process in any way! Meanwhile, she’s now on The View every day, giving an air of legitimacy to her form of pseudoscience. And the movement she bolstered is causing huge numbers of kids to go unvaccinated, get sick and sometimes die of preventable diseases—and, even worse, spread those diseases to and often kill infants too young for vaccination (even if their parents do believe in modern medicine).

All of this nonsense persists largely because these yuppie anti-vaxxer parents get all their info from bad websites rather than from trained medical professionals. They’re too young to remember the bad old days when measles, mumps, rubella, whooping cough, chicken pox, and especially polio routinely sickened thousands of kids every year, and quite a few of them died from these preventable diseases. I remember them vividly when I was growing up in the 1950s, and I got very sick on several occasions. My mother (now 90 years old) suffered from polio, which traumatized their entire family for years, even though she survived with only minor muscle loss in her abdomen.

A recent photo of Jenny McCarthy and her son Evan

A recent photo of Jenny McCarthy and her son Evan

The emotional and family trauma of finding out your child has autism leads them to all sorts of irrational action and quack cures, because the truth is not what they want to hear: there is no “cure” for autism since it is largely genetically inherited, so NOTHING they did or didn’t do during the child’s lifetime caused it. Instead, the fault lies in their own genes, which they cannot change or control.

Nevertheless, you hear the anti-vaxxers go on and on about “my son got his shots, then we first noticed the symptoms of autism.” This the famous fallacy of false correlation, or correlation is not causation. Just because two events happen to co-occur does not mean that one caused the other, or even that one is related to the other in any way.

But some might say: “Where there’s smoke, there’s fire” (another false maxim, which assumes guilt when nothing is proven). There is no scientific or medical evidence connecting autism and vaccines, just gut reactions, anecdotes, and emotions of the anti-vaxxers.  But what might explain the possible association in the first place? Why does it appear than autism increased at about the time vaccines became widespread?

As in the case of many examples of false correlation, there is usually another unrelated cause that  explains it. It turns out that children at about 18 months old reach a critical stage in their development. This is when they form simple sentences, and move from chewing or pawing toys to interactive forms of play. This is also the age where a child who might already have inherited autism genetically will begin to show developmental delays, and thus parents would notice their child’s autism for the first time. And it so happens that this is the age when the MMR vaccines are traditionally given. Once again, two unrelated events that happen to coincide in time might explain this false attempt at a causal connection.

What about the increasing rates of autism in children over the past few years? Once again, when you look closely at the data, this assertion breaks down. Contrary to the anti-vaxxers’ claims, the increase in documented cases does not precisely track the timing of the introduction of certain vaccines, but lags some years behind it. More importantly, the increase in reported cases is widely acknowledged by the medical community to be an artifact of the historical reality that autism was not even a recognized disorder in psychiatry until the late 1960s. Naturally, once a disorder was formally diagnosed and named, it was gradually recognized and diagnosed by more and more doctors as it became more familiar. In recent years, child psychologists have developed even more sophisticated tests to spot it early, leading to even more diagnosed cases. Prior to the formal diagnosis of autism, there were many cases of people with this disorder, but they were just lumped into the “mentally retarded” and their diagnosis was incorrect: they were called “adult psychopaths” or “schizophrenics”. Once again, correlation does not prove causation. Here, the increase in both diagnoses and vaccinations is an artifact of two unrelated events that occurred as medicine improved in the 1960s and the 1970s: the increase in childhood vaccinations, and the improved ability to diagnose different psychological disorders.

As the blogger Dr. David Gorksi, AKA Orac (a surgical oncologist) argued about the false correlation between autism and the rise of vaccinations in 1983:

A lot of other things have happened since 1983 as well. For example, in the early 1990s, the diagnostic criteria for autism were broadened, and campaigns for greater awareness were begun. Diagnoses of autism in 1983 were made using the DSM-III, where the criteria for an autism diagnosis were much more restrictive than those in the DSM-IV, released in the early 1990s. Moreover, in 1983, categories of Asperger’s and pervasive developmental disorder—not otherwise specified, both of which are lumped into the 1 in 150 figure for 2008, weren’t recognized in the DSM-III. Of course, if I wanted to be snarky (and perish forbid that I would ever be snarky), I could point out that 1981 was the year that the IBM PC was released, followed by the Apple Macintosh in 1984, both of which led to the exponential growth of households owning and using personal computers. That’s it! It must be computer use that led to the increase in autism in the 25 years since 1983! Wait, what about the compact disc? It just so happens that 1983 is the year that the CD was first released in the American market. Ergo, it must be CDs that cause autism.

The apparent false correlation between the rise of autism and the expansion of health food consumption in the 1980s

The apparent false correlation between the rise of autism and the expansion of health food consumption in the 1980s

We can take the ridiculous false correlation example even further. For example, not only sales of CDs or personal computers has increased since the early 1980s, but many things have. For example, the sales of health foods increased over the same period! Does health food cause autism? Of course not! Their expansion just happens to be associated in time. In another irony, the anti-vaxxers are often big advocates about how certain health foods will magically “cure” diseases. If they believe that “correlation proves causation,” they’d better change their diets and stop going to Whole Foods Market.

Or here’s a real zinger: the rise of autism actually correlates with the rise of Jenny McCarthy’s career! AHA! So Jenny herself MUST have caused autism to increase in America. Get thee to a nunnery, Jenny!

Clearly, this plot proves that Jenny McCarthy herself caused the rise in autism!

Clearly, this plot proves that Jenny McCarthy herself caused the rise in autism!

23 Responses to “Autism and vaccines: correlation is not causation”

  1. Daniel Bastian says:

    Excellent piece. However, I noticed at least three broken links above. What happened there?

  2. J Greenfield says:

    I believe your piece is misleading, and doesn’t like many articles on the subject of autism take into account the amount of shots (vaccines) given to a 1 year-old child in a single day. Actually the whole debate seems to be to be about about individual vaccine safety – only. Is autism largely “genetically” inherited? Sure, but in what numbers? I do agree that the “spectrum” has widened, and therefore many more children are in the “net”. But until an honest and proper primate study is done, I have a strong suspicion that the amount of viruses given on a single day, to a young child without a fully developed immune system is dangerous. And I’m not just talking about the MMR; that’s only 3. That may partially explain why the curve of autism increased with “more” vaccinations.

    I for one believe the shots (vaccines), when given one at a time, pose zero, or minimal threat. But when grouped together in one day, at the one year visit ( Hib, pneumococcal, chicken pox (varicella), MMR, and hepatitis A vaccines — not to mention the hepatitis B, and polio vaccines if the child hasn’t had his/her 3rd doses yet), I believe these shots combine a “powerful punch” to 1 year-old’s immune system, and can potentially “flip a switch” and alter a child, I.E. autism. I’m 43 years old and only had 5-6 vaccines in my first 5 years; and yes, most likely the “big” and “bad” MMR made with mercury.

    We are very fortunate to have a Pediatrician who is working with us as we’re using a slower vaccine schedule. What shocks me is how many Pediatricians just follow and defend the American Academy of Pediatrics vaccine schedule, and don’t dare question, or alter it.

    When I knew that I was going to be a parent for the first time, all of this talk about vaccines and autism interested me a great deal. I researched, listened to Doctors, read the Dr. Sears book, etc. And finally found a little primate study by the University of Pittsburgh where baby monkeys also developed autism when given the same shot schedule. That study has been discredited because the lead investigator, Laura Hewitson, herself had a claim with the vaccine court on behalf of her own child at the time of her study. Discredited? Of course!

    There’s a ton of money at stake between drug companies, the FDA, CDC and Academy of Pediatrics, etc . Whether the original study, or more recent studies are correct, or not, doesn’t really matter at this point. What matters is the fact that the FDA, CDC and the Academy of Pediatrics haven’t themselves done, or demanded an extensive primate study which focuses on the vaccine schedule. Why isn’t anyone looking at, or demanding this? As you write, “genetic factors” are to blame, but then why did the number go from 1 in 5000, to 1 in 88 American children over the past 20 odd years? Is the spectrum to blame? Sure, a bit. What about environmental factors? Yes, that too. Correlation is not causation. But be careful. Something changed 20 odd years ago; the shot schedule. Children today are getting more than 30 shots/vaccines in their 1st year. Don’t you think “someone” should be looking at that?

    • You apparently didn’t understand the blog. If ASD is largely genetic, NO environmental factor (not shots, how often they are given, etc.) will make ANY difference! It’s in your genes, NOT in your shots. And I also pointed out why the correlation with shots is completely meaningless: the ultimate cause is the expanding definition of ASD, and improved recognition and diagnosis. The correlation with the change of the shot schedule makes no more difference that the correlation with health food or the correlation with Jenny McCarthy’s career.

    • Chris says:

      “I’m 43 years old and only had 5-6 vaccines in my first 5 years; and yes, most likely the “big” and “bad” MMR made with mercury.”

      One of the big indicators that someone does not know the issues is when they claim that the MMR vaccine actually contained thimerosal.

      Plus have you really skipped tetanus boosters? Wow, just wow.

      You might want to do a bit more research. Here is one place to start: Vaccine Safety: Examine the Evidence.

    • Chris says:

      “What matters is the fact that the FDA, CDC and the Academy of Pediatrics haven’t themselves done, or demanded an extensive primate study which focuses on the vaccine schedule. Why isn’t anyone looking at, or demanding this?”

      I just need to add that someone who has made egregious errors about some vaccines does not get to demand wasting the lives of primates.

      Of course the Hewitson studies were discredited, they were done very poorly. Control animals disappeared from the data (researchers are not allowed to cherry pick, it is very bad form). Plus she never disclosed her conflict of interest about being a vaccine court litigant.

  3. Ichthyic says:

    FWIW, the vast majority of parents thinking that autism is linked to any specific cause are usually suffering from recall bias:

    • Max says:

      That’s how you attribute most things to a non-obvious cause. You hear that X causes Y. Then, when Y happens, you recall whether X happened. e.g. You come down with flu and recall where you might have caught it. Must’ve been that coworker who was coughing.

  4. Ichthyic says:

    Now she and her anti-vaxx cohort are demanding that medicine and drug companies should “green the vaccine”!

    Just the other day, in a thread where people were commenting on something really dumb an antivaxxer said (never get tired of THAT, eh?), I saw this:

    > This is the THIRD time today I’ve read and heard
    > a anti-vaxxer say vaccines have DOG CELLS in them.
    > Is this the new vaccine ingredient for 2014?

    Dog cells? never head that one before, so, using basic Google fu, i simply searched on it.

    Yes, turns out antivaxxers are now pushing the idea that people are “tainting” themselves with vaccines that have “dog cells” and “worm cells” in them.

    Now, I know for a fact that no vaccine has live cells in it, so something was seriously wrong with this statement, so another google search that took all of ONE MINUTE, turned up what goes on here.

    Vaccines have been traditionally developed using egg cells, namely, chicken eggs (the eggs cells serve as vaccine substrate – look it up). Turns out, some people are indeed allergic to chicken eggs (albumin proteins – there of course are not chicken eggs in vaccines either), so a few years back, efforts were made to find a suitable substrate that was NOT of avian origin.

    Dog kidney cells were selected and tested, among other choices, and found to be even more reliable than chicken egg cells for substrate production. Also found to be more hypoallergenic, that is SAFER… than chicken eggs.

    so, no you don’t get “dog cells” in a vaccine, just like you don’t get “chicken eggs” in a vaccine, but some of the proteins from the cells end up in minute quantities in the vaccines.


    I’m typing that in all caps not just to call attention to it, but because I really, REALLY REALLY hate antivaxxer bullshit.

    [Edit: let me make that even more clear. I hate antivaxxer reasoning, which is nothing but confirmation and recall bias, compiled with lies and misinformation. But antivaxxers that persist in the face of knowing there are innumerable counters to their inane points? I hate THEM too. Seriously, if you are an antivaxxer, and proud of it... don't EVER try to talk to me. You won't like me much at all. one and only warning.]

    it has already cost more lives than all the recorded instances of allergic reactions to vaccines combined, and then some.

    here’s one of many papers published showing that using dog kidney cells as a substrate is more reliable and safer than chicken eggs:

    • tmac57 says:

      Now see,this is where my natural instinct as a dog lover would make me have just the opposite reaction. I would be like “Hey! I got me a guard dog vaccine protecting me from a scary disease!”
      Good doggy! :)

  5. Max says:

    Typical experience: “Her autism symptoms began not at birth but gradually appeared after each vaccination, most markedly after her MMR vaccine, when she broke out with a full body rash, fever, lost eye contact and then her words stopped.”

    Are these actual MMR vaccine side-effects? It’s obvious how a parent would associate the last two with the child developing autism.
    Or is the loss of eye contact and communication just a coincidence? In that case, it should correlate just as well with any random day of the year.

  6. Max says:

    If you point out the correlation between the introduction of vaccines and the drop in the incidence of diseases like smallpox, measles, and polio, anti-vaxxers will say that correlation is not causation.

    • tmac57 says:

      Yes they do,and it fools some people because while literally being true,it fails to mention that we know the mechanism by which vaccines work,so there is solid plausibility,and we can easily see as they are methodically deployed,the incidence of the disease that they target start to decline,and in the case of smallpox,and potentially polio,they are boxed in and eradicated from the population.
      The antivaxers like to say those decreases are due to better hygiene and such,but that just flies in the face of the evidence.

      • Chris says:

        Yet they have so much trouble answering my question of why the incidence of measles dropped 90% between 1960 and 1970 in the USA.

    • markx says:

      Many seem to forget that commercial animal vaccines have been rigorously tested in challenge trials: ie, two matched groups of animals are put in a common area, one group is vaccinated against disease x, the other group is not. Both groups are then deliberately and precisely challenged with the live disease x pathogen. Clinical signs, growth parameters, morbidity, and mortality are monitored.

      Results show very, very clearly that vaccines can give almost miraculous levels of protection against a huge range of serious diseases in a large number of species.

      There are also huge amounts of human epidemiological data showing differences in disease incidence and severity between vaccinated and unvaccinated populations.

      There is no doubt that vaccines work. There is almost no credible evidence to support the idea that the current human vaccination regime causes the ‘upsurge’ in autism cases. There is plenty of evidence that understanding and diagnosis of autism has accelerated hugely over the last two decades. However, given the concerns, further research on the effects of the current vaccination regime may perhaps be warranted. Certainly research into the causes of autism are ongoing.

  7. Brandon Shollenberger says:

    While I largely agree with the criticisms of anti-vaxxers, I don’t think it’s quite fair to say they “have no clue what mainstream science” is about. Mainstream science can abuse correlations as badly as any anti-vaxxer. In fact, it can do it even worse. Look at the sort of things psychological research comes up with. It says the opposites of two things are correlated therefore those two things are related. That’s almost like looking at a group of people who don’t have cancer and haven’t been hit by a car then concluding getting hit by a car causes cancer.

    I believe medicinal science is a far better field, but if accepted, mainstream science can abuse correlations worse than anti-vaxxers, shouldn’t we be a bit more sympathetic toward them? How are they supposed to know what fields tolerate, or even promote, quackery?

    • Max says:

      If people who don’t have cancer are less likely to be hit by a car, then that’s the same as saying that people who do have cancer are more likely to be hit by a car, which means those two things are somehow related.
      If Global Warming deniers are more likely to deny the moon landing, then those things are also related, perhaps by a general distrust of government and mainstream science.

  8. Nelsonneitzche says:

    “Frankly, I doubt that a source like Time magazine misquoted her” why not?

  9. zardac says:

    You prattle on while recklessly condemning one particular group- yuppies.

    Apparently you believe the poor, the working classes, dinks, hipsters, and the Amish are less likely to shun vaccines?


    I do agree Jenny McCarthy deserves our scorn, and unlike other purveyors of nonsense (such as John Edwards) Ms. McCarthy’s shtick is likely to directly endanger children.

    Sadly, Autism seems serve as a magnet in attracting hucksterism.

    In the time I’ve parented an autistic child, I’ve witnessed the giddy embrace and eventual discrediting of interventions such as secretin therapy and facilitated communication-and I know other nonsensical treatments continue to be widely popular.

    This may be a result of “emotional and family trauma (leading to) all sorts of irrational action and quack cures” but this isn’t an inevitable family dynamic.

    We’ve been able to tell our kid that we consider Asperger syndrome to be more of a difference than a disability, and we love him just the way he is…We’re not in the market for a cure.

  10. Meri says:

    Vaccines do not cause Autism, there are no statistics that support it. I think chemicals have increased the prevalence of Autism Spectrum Disorders. I am almost positive I read somewhere that there was a correlation between aluminum and Ahlzheimer’s. There must be something causing the huge increases in Autism diagnoses over the past decade.

    • If you read the article carefully, NOTHING in the environment causes autism–it’s largely genetically inherited. And as the article clearly states, we KNOW what the cause in increased ASD diagnoses is: the definition and improved ability to diagnose ASD in the medical and psychological profession. No other factor need be invoked!

  11. Seth Bittker says:

    Autism typically features a Th2 skew to the immune system which means allergic response is up regulated. So to me it is plausible that some kids who develop autism have reactions to vaccines. A logical question is why these kids have a Th2 skew to the immune system which predisposes them to a pathological allergic response to vaccines (as well as other allergens such as casein and gluten)?

    One answer could be genetics. Indeed if I am reading your views correctly you are saying genetics is the sole factor in autism. I am skeptical that this is the sole factor in most cases. To me the increasing incidence of autism seems all to real and genetics would take generations to play out. This suggests that there is some change in how we raise our kids which is responsible for up regulating allergic response. One factor that has changed in the past few decades is that we give our babies larger and larger amounts of vitamin D. Interestingly vitamin D induces a Th2 skew to the immune system and the biochemical markers that one typically sees in autism are similar to biochemical markers that one would expect from excess supplementation,

    • A large percentage of ASD is inherited; we just don’t know how much. Some say as much as 90%; others say less. Here’s a recent link to some of the research:
      But if heritability is anywhere in the 70-90% range, as most studies suggest, then the environment plays a VERY minor role (IF ANY) is the appearance of ASD. THus, it has nothing to do with vaccines, diet, etc. any more than it has to do with the spread of personal computers or the development of the CD or the rise of Jenny McCarthy’s career.