Nicholas Kristof on toxins and autism

Kristof is correct to note:

Frankly, these are difficult issues for journalists to write about. Evidence is technical, fragmentary and conflicting, and there’s a danger of sensationalizing risks.

But he falls into these very traps when suggesting that toxins play a major role in autism.  Let me pick on two sentences.  Try this one:

There are genetic components to autism (identical twins are more likely to share autism than fraternal twins), but genetics explains only about one-quarter of autism cases.

Kristof doesn't note that identical twins both are autistic ninety percent or more of the time (conditional on one of the twins being autistic), yet the concordance is much lower for fraternal twins.  That militates in favor of genetic explanations, although the mechanics of transmission are poorly understood.  It's wrong to cite genetics as explaining one-quarter of autism cases or to imply that genetics do not explain three-quarters.  There are recent studies which look for correlated genes across autistics and find less than overwhelming results and perhaps this is what he has in mind.  More accurately, there is a common problem with finding "simple" genetic markers for traits which are very likely or even certain to be genetic.  The degree of correlation across genetic patterns we can find should not be taken as a measure of how many autistic cases — or any other condition — can be explained by genetics.  By the way, here is one paper with a plausible genetic model of autism.

Kristof also writes: "Of children born to women who took valproic acid early in pregnancy, 11 percent were autistic."

Probably he is referring to Moore (2000), "A Clinical Study of 57 Children with Fetal Anticonvulsant Syndrome."  A total of four (supposedly) autistic children were observed to produce this conclusion.  What happened is that some mothers took a potentially dangerous substance during pregnancy, many of their children had problems — of a variety of kinds – and some of these problems ended up resembling some features of autism or at least were interpreted as such.  It's unlikely those were four autistic children in the classic sense.  The paper also gives no real information on its standard of diagnosis for autism or what it means by autistic traits.  It's common that papers like this find some problems in children and simply call those children "autistic," then leaping to false overall conclusions. 

There's also a paper on using valproic acid to treat autism.  One possibility is that the mothers taking valproic acid already were more likely to have autistic children; more likely our entire body of knowledge on valproic acid and autism doesn't offer real information.

Cross-sectional studies, spanning decades of age groups, suggest a roughly constant rate of autism, even when environmental toxins are changing considerably over those lengthy time periods.  Plenty of other studies relate autism clusters successfully to non-toxin factors, such as parental education or supply-side services or standards of diagnosis.

There are likely well over 50 million autistics in the world and most of them have not had significant exposure to the cited toxins.  While there are some plausible heterogeneities within autism, it is necessary to ask whether "genes *or* toxins" is one of those and probably it is not.

Epigenetic factors have not been ruled out in autism but the most careful discussions recognize that the relevant epigenetic factors — if indeed any are important – are unknown and also need not fit our usual intuitions about what is harmful in terms of direct dosages.  A different way to approach the question is to ask which environmental features raise the rate of mutation.  That way the genetic and epigenetic explanations are at least potentially consistent.

I'm not defending the feeding of "toxins" to children, but on examination I think virtually all of the major specific claims in this Op-Ed — at least those about autism — are wrong.

Addendum: David Bernstein scores some telling points.


"identical twins both are autistic ninety percent or more of the time" - surely conditional upon one twin being autistic, right?

A very good grasp of biological science (I hesitate to add "for an economist" because I don't know your specific background)! I find too many public consumption writers making the mistakes of the op-ed critiqued here and too few arguing, correctly, along your lines of critique. As a breast and ovarian cancer researcher I am always confronted with X causes ovarian cancer or Y prevents it, when in reality it is almost never that cut and dry.

We could also be looking at genetic susceptibility to specific uterine influences or toxins. This will be more specific to sort out. A study with 57 patients should not be used to draw conclusions about millions of people.


surely conditional upon one twin being autistic, right?

He probably means that if one member of a pair of identical twins is autistic, in 90% of cases the other twin is autistic as well.

That definitely sounds genetic in basis, which presumably isn't much comfort to the families with autistic children.

tyler, your link to the twin study abstract says "In the combined sample 60% of monozygotic (MZ) pairs were concordant for autism versus no dizygotic (DZ) pairs."

the 92% figure was for "a broader spectrum of related cognitive or social abnormalities"

hardly a slam dunk for an exclusively genetic cause.

Landrigan has a paper in The Lancet on the subject. Is that a 'leading' journal?

It is remarkable that neurons create the correct connections most of the time. When this doesn't succeed strange things happen.

For example, an apparent mis-wiring of neurons has left the guy over at Volokh with the ability to speak out of his ass.

I am actually puzzled why someone like Kristof still gets op-eds to write about subjects he knows nothing about. I had hoped blogs would have killed op-ed "journalism" by now. What's the hold up?

I agree, plastics = toxins was wrong and was used for cognitive fluency. Plastics come in multitudes of varieties and some "toxins" are useful too.
Plastics are immensely useful but why not first focus on the plastics used in the kitchen and improve the scientific knowledge of the public one step at a step.
Generalizing things for cognitive fluency is wrong but its also wrong to vindicate everything that is abstract because of difficulties in nomenclature and the innate slowness in segregating factors out of abstraction via scientific research.
Should we consider some "environmental factors" innocent until proven guilty? Why do torts thrive in this society?
Our sound byte driven society doesn't think much about the importance of science and scientists. But at the same time just because earning a living scientifically doesn't give one an innate right to call others names even if they are wrong.
This mutual aversion between two classes of society should stop. Science is much more important than flexing cognitive proness. There is no us vs them.
It' obvious Kritsoff didn't find any concrete proof that chemicals in plastic is causing autism but does anyone else have concrete proof that chemicals in plastic doesn't cause autism?
Who pays the price of this deadlock?

Ummm.... heritibility is derived from twin concordence rates. Heritibility is the proportion of a traits variability due to genetic differences. It is not on the face of it incorrect to state that 25% of variation is due to genetic differences (the number is a sta that would require knowing the frtarnal twin concordence rate). However, most heritibiltity estimates are higher than .25 for autism. Readers should note that any concordence rate less than 100% (even 90% concordence rate) among gentically identical clearly rules out that genes fully account for who does and does not get autism. Genbes matter, but it is not al genes. This is not in dispute. It is of at least some interest that only one study found such a high concordence rate, anyway. Wikipedia has a great overview for heritibility, FYI.

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