Environmental influences on autism - splashy headlines from dodgy data

A couple of recent papers have been making headlines in relation to autism, one claiming that it is caused less by genetics than previously believed and more by the environment and the other specifically claiming that antidepressant use by expectant mothers increases the risk of autism in the child. But are these conclusions really supported by the data? Are they strongly enough supported to warrant being splashed across newspapers worldwide, where most readers will remember only the headline as the take-away message? The legacy of the MMR vaccination hoax shows how difficult it can be to counter overblown claims and the negative consequences that can arise as a result.

So, do these papers really make a strong case for their major conclusions? The first gives results from a study of twins in California. Twin studies are a classic method to determine whether something is caused by genetic or environmental factors. The method asks, if one twin in a pair is affected by some disorder (autism in this case), with what frequency is the other twin also affected? The logic is very simple: if something is caused by environmental factors, particularly those within a family, then it should not matter whether the twins in question are identical or fraternal – their risk should be the same because their exposure is the same. On the other hand, if something is caused by genetic mutations, and if one twin has the disorder, then the rate of occurrence of the disorder in the other twin should be much higher if they are genetically identical than if they only share half their genes, as fraternal twins do.

Working backwards, if the rate of twin concordance for affected status are about the same for identical and fraternal twins, this is strong evidence for environmental factors. If the rate is much higher in monozygotic twins, this is strong evidence for genetic factors. Now to the new study. What they found was that the rate of concordance for monozygotic (identical) twins was indeed much higher than for dizyogotic (fraternal) twins – about twice as high on average.

For males: MZ: 0.58, DZ: 0.21
For females: MZ: 0.60, DZ: 0.27

Those numbers are for the diagnosis of strict autism. The rate of “autism spectrum disorder”, which encompasses a broader range of disability, showed similar results:

Males: MZ: 0.77, DZ: 0.31
Females: MZ: 0.50, DZ: 0.36.

These numbers fit pretty well with a number of other recent twin studies, all of which have concluded that they provide evidence for strong heritability of the disorder – i.e., that whether or not someone develops autism is largely (though not exclusively) down to genetics.

So, why did these authors reach a different conclusion and should their study carry any more weight than others? On the latter point, the study is significantly larger than many that have preceded it. This study looked at 192 twin pairs, each with at least one affected twin. However, some recent studies have been comparable or even larger: Lichtenstein and colleagues looked at 117 twin pairs and Rosenberg and colleagues looked at 277 twin pairs. These studies found eveidence for very high heritability and negligible shared environmental effects.

Another potentially important difference is in how the sample was ascertained. Hallmayer and colleagues claim that their assessment of affected status was more rigorous than for other studies and this may be true. However, it has previously been found that less rigorous assessments correlate extremely well with the more standardised assessments, so this is unlikely to be a major factor. In addition, there is very strong evidence that disorders like autism, ADHD, epilepsy, intellectual disability, tic disorders and others all share common etiology – having a broader diagnosis is therefore probably more appropriate.

In any case, the numbers they came up with for concordance rates were pretty similar across these studies. So, why did they end up with a different conclusion? That’s not a rhetorical question – I actually don’t know the answer and if anyone else does I would love to hear it. Given the data, I don’t know how they conclude that they provide evidence for shared environmental effects.

The methodology involves some statistical modeling that tries to tease out the sources of variance. However, this modeling is based completely on a multifactorial threshold model for the disorder - the idea that autism arises when the collective burden of individually minor genetic or environmental insults passes some putative threshold. Sounds plausible, but there is in fact no evidence - at all - that this model applies to autism. In fact, it seems most likely that autism really is an umbrella term for a collection of distinct genetic disorders caused by mutations in separate genes, but which happen to cause common phenotypes (or symptoms).

If that is the case, then what the twin concordance rates actually measure is the penetrance of such mutations – if one inherits mutation X, how often does that actually lead to autism? For monozygotic twins, let us assume that the affected proband (the first twin diagnosed) has such a mutation. Because they are genetically identical, the other one must too. The chance that the other twin will develop autism thus depends on the penetrance of the mutation – some mutations are more highly penetrant than others, giving a much higher probability of developing a specific phenotype. If we average across all MZ twin pairs we therefore get an average penetrance across all such putative mutations. Now, if such mutations are dominant, as many of the known ones are, then the chance that a dizygotic twin will inherit it is 50%, while the penetrance should remain the same. So, this model would predict that the rate of co-occurrence in DZ twins should be about half that of MZ twins, exactly as observed. (No stats required).

The conclusions from this study that the heritability is only modest and that a larger fraction of variance (55%!) is caused by shared environment thus seem extremely shaky. This is reinforced by the fact that the confidence intervals for these estimates are extremely wide (for the effect of shared environment the 95% confidence interval ranges from 9% to 81%). Certainly not enough to overturn all the other data from other studies.

What about epidemiological studies that have shown statistical evidence of increased risk of autism associated with a variety of other factors, including maternal diabetes, antidepressant use, season and place of brith? All of these factors have been linked with modest increases in the risk of autism. Don’t these prove there are important environmental factors? Well, first, they don’t prove causation, they provide a statistical evidence for an association between the two factors, which is not at all the same thing. Second, the increase in risk is usually on the order of about two-fold. Twice the risk may sound like a lot, but it's only a 1% increase (from 1 to 2%), compared with some known mutations, which increase risk by 50-fold or more.

The main problem with these kinds of studies (and especially with how they are portrayed in the media) is that they are correlational and so you cannot establish a causal link directly from them. In some cases, two different correlated parameters (like red hair and freckles, for example) may actually be caused by an unmeasured third parameter. For example, in the recently published study, the use of antidepressants of the SSRI (selective serotonin reuptake inhibitor) class in mothers was associated with modestly increased risk of autism in the progeny. This association could be because SSRIs disrupt neural development in the fetus (perfectly plausible) but could alternatively be due to the known genetic link between risk of depression and risk of autism. Rates of depression are known to be higher in relatives of autistic people, so SSRI use could just be a proxy for that condition. The authors claim to have corrected for that by comparing rates of autism in the progeny of depressed mothers who were not prescribed SSRIs versus those who were but one might imagine that the severity of depression would be higher among those prescribed an antidpressant. In addition, the authors are careful to note that their findings were based on a small number of children exposed and that "Further studies are needed to replicate and extend these findings". As with many such findings, this association may or may not hold up with additional study.

As for season and place of birth, those findings are better replicated and, interestingly, also found for schizophrenia. There is a theory that these effects may relate to maternal vitamin D levels, which can also affect neural development. This also seems plausible enough. However, the problem in really having confidence in these findings and in knowing how to interpret them is that they are population averages with small effect sizes. Overall, it seems quite possible that the environment - especially the prenatal environment - can play a part in the etiology of autism. At the moment, splashy headlines notwithstanding, genetic factors look much more important and genetic studies much more likely to give us the crucial entry points to the underlying biology.

Hallmayer J, Cleveland S, Torres A, Phillips J, Cohen B, Torigoe T, Miller J, Fedele A, Collins J, Smith K, Lotspeich L, Croen LA, Ozonoff S, Lajonchere C, Grether JK, & Risch N (2011). Genetic Heritability and Shared Environmental Factors Among Twin Pairs With Autism. Archives of general psychiatry PMID: 21727249

Lichtenstein P, Carlström E, Råstam M, Gillberg C, & Anckarsäter H (2010). The genetics of autism spectrum disorders and related neuropsychiatric disorders in childhood. The American journal of psychiatry, 167 (11), 1357-63 PMID: 20686188

Rosenberg, R., Law, J., Yenokyan, G., McGready, J., Kaufmann, W., & Law, P. (2009). Characteristics and Concordance of Autism Spectrum Disorders Among 277 Twin Pairs Archives of Pediatrics and Adolescent Medicine, 163 (10), 907-914 DOI: 10.1001/archpediatrics.2009.98

Croen LA, Grether JK, Yoshida CK, Odouli R, & Hendrick V (2011). Antidepressant Use During Pregnancy and Childhood Autism Spectrum Disorders. Archives of general psychiatry PMID: 21727247


  1. Still, gene studies have no true candidate gene. There is no reason environment should not be looked at.

  2. Actually there are over one hundred genes or chromosomal loci with mutations confirmed to cause autism. See reviews by Catalina Betancur or myself. Each of these is very rare but they so far collectively explain a quarter of all autism cases and that number continues to grow as more are being found almost weekly. So we are well beyond candidates at this stage.

  3. I think there are two pieces of the twin study that you might not be considering.

    The first is that the pairwise concordance estimates for the MZ twins in the current study are lower, although maybe not significantly, than other recent estimates.

    For example, Rosenberg et al in 2009 showed that for broad ASD the overall concordance for MM MZ twins was about 86% whereas the FF MZ was 100%. Interestingly, both studies had the same number of FF MZ twin pairs (9) but came up with very different results.

    The second and more important part has to do with the difference between DZ twins and non-twin siblings. Both Rosenberg and the current study showed a much higher concordance between DZ twins than other estimates in the past. These new estimates mean that DZ twins are much more likely than two other non-twin siblings to both have autism.

    Since the major difference between DZ twins and non-twin siblings is that they are born at the same time and thus share a similar prenatal and early childhood environment, this difference means that the shared environments during this time plays a much larger role than does the shared genetics.

    If the opposite was true - that environmental influences were very small - then DZ twins should have almost the same chance as non-twin siblings to both have autism.

    For reference, the chance that multiple siblings both have autism is likely somewhere under ten percent (the CDC's data page says 2% to 8%). Although I think I have seen estimates as high as 20% for an extremely broad definition of autism. The jump from lets say 10% for non-twin siblings to 25% or 30% for DZ twins has to come from the shared environment or other factors related to being a twin (more on that is a minute).

    When you put those two ideas together - that shared genetics alone isn't enough to force MZ twins to always both have autism (or even the same severity when they do) but that the shared environment of DZ twins is enough to make then much more likely to both have autism than non-twin siblings - you get the idea that the environment plays a large role.

    Although, there could be other explanations here as well. Twins of all types are more likely to have prenatal problems, are (typically) smaller at birth, and more likely to be born premature than singletons. All three of those factors are thought to be risk factors for autism. DZ twins are more likely to be born to older mothers and the number of DZ twins being born to older mothers is increasing (faster then births to older mothers in general, at least in CA).

  4. MJ, you are right that if there is a large difference between the recurrence risks for DZ twins compared to full sibs that that would indicate a role for the prenatal environment. The Hallmayer study does not appear to rely on such a difference for its modeling, however - at least they don't state that they do, as far as I can tell. The full sib concordance rates that I have seen start at 10% for full autism but add another 20% for broader diagnoses (up to 30% total). For a proper comparison these should really be looked at in the same study.

    Also, there is one other potentially important factor that may make DZ twin concordance higher than full sibs, which is the phenomenon of "stoppage", where parents with one autistic child choose not to have any others. This means the likelihood of an autistic child's older siblings being autistic is greatly reduced. This obviously cannot happen with twins.

    All that said, I am quite ready to believe there are important prenatal environmental influences - after all autism is a neurodevelopmental disorder, so other things besides genetic mutations that disturb it may well have the same result. I just don't think the data in this paper make as strong a case for them as the authors claim.

  5. Kevin,

    You are correct that the Hallmayer paper does not directly address the non-twin sibling issue but I don't think that is a bad thing per se. I think it is better for a paper to ask a single question and answer that question well rather than try to address multiple questions and not to address any of them properly. So while it would be better if there was an actual value for non-twins in the study, it would have required an entirely different set of data, a different analysis, and would have diluted the twin part.

    However, there are hints in the paper to what the sibling rate would look like in the form of the MF DZ twin rates. If you notice, those concordance rates are much less than all of the same sex twins - even less than you would expect after taking into account the relative risks for males and females (4 to 1 ratio). If you also take into account that these are all DZ twins with the addition risk factors that entails (older mothers, prenatal complications, premature, etc) , I think it would be safe to conclude that non-twin siblings would have fallen in this range.

    But even without that, there are other estimates of sibling reoccurance out there that you can compare to. For example, there is the 2% to 8% in the 2005 study quoted by the CDC's data page (http://www.cdc.gov/ncbddd/autism/data.html). And then there is the the high risk baby sibs project of Autism Speaks that suggest that siblings of children with autism have a ten times greater risk than other families (which would imply about 10%).

    Perhaps the comparison isn't quite apples to apples but since the current study is using a very conservative definition of autism (ADOS and ADI-R have to agree), I think it would be safe to draw some conclusions.

    As for the issue of stoppage, I think you are right up to a point. Parents would be much less likely to have another child after they find out that an earlier one has autism. But since that typically doesn't happen until after the second or third birthday (or at least didn't happen until then historically) and most siblings are conceived within 3 years (I believe it is like 80%), there is an adequate window for siblings to happen even with stoppage. Maybe not in the more severe cases when the signs of autism are clearly visible even at a very age, but certainly in the the less severe ones such as the broad definition of autism used in this study.

    As an aside, I don't think I have ever seen a paper that suggested that non-twin siblings have a total 30% reoccurrence rate when using the ADOS and/or ADI-R. I know I have seen ones that said that autistic traits might be that common using something like the AQ or SCQ, but it is a long way from traits to an actual diagnosis. If you had a reference that showed that using the ADOS,ADI-R, or even the DSM-IV criteria instead of a screen, I would interested in reading it.

  6. Bearing in mind the extremely dodgy basic definition of autism. Remember, the chinese menu definition techniques of DSM-IV - two out of these five combined with one out of these three and so on - leads by simple perms and combs to the conclusion that there are approximately 12,000 ways to be diagnosed with autism. I'm not contesting that there are problems, but the evidence that it's a single problem is hardly compelling. Which does go some way to explaining why in California the best predictor of being diagnosed with autism is your distance from an autism centre.

  7. Not to beat a dead horse, but I ran across this snippet in a paper on the genetics of autism that was just published this week. As I understand it, this was the accepted profile of autism for MZ twins, DZ twins, and non-twin siblings up until the Hallmayer paper was published. I have seen similar language in quite a few papers over the past few years.

    "The prevalence of ASD is between 10 in 10,000 and 60 to 70 in 10,000, depending on the precise definition used [3]. Evidence from family studies implies that ASD has a strong genetic basis: the concordance rate in monozygotic (MZ) twins ranges from 70 to 90%, whereas dizygotic (DZ) twin concordance varies from 0 to 10% [4, 5]. Familial aggregation studies have shown that the relative risk of developing autism in firstdegree relatives of an autistic patient is 3 to 7%, which is ten-fold higher than the prevalence in the general population."

    1: Gillis RF, Rouleau GA. The ongoing dissection of the genetic architecture of
    Autistic Spectrum Disorder. Mol Autism. 2011 Jul 8;2(1):12. [Epub ahead of print]
    PubMed PMID: 21740537.

    The full text is available here : http://www.molecularautism.com/content/pdf/2040-2392-2-12.pdf

    Now plug the results from Hallmayer into the numbers above and you can see what the big deal is. That almost 10 fold jump in risk from non-twin siblings to fraternal twins strongly implies that the prenatal or early childhood environment matters quite a bit.

    I realize that it isn't an apples to apples comparison but I think the comparison makes it clear just why Hallmayer is getting a lot of press.

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  9. two out of these five combined with one out of these three and so on - leads by simple perms and combsxlpharmacyafwetweI think the comparison makes it clear just why Hallmayer is getting a lot of press.


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