For over two decades now, so-called “autism experts” have been claiming that autism is more than 90% caused by genes. The influence of these claims on autism policy and research funding is hard to overstate. But few realize that the basis of these claims hangs on a fragile evidence base: two small twin studies--one from Great Britain, the other from Scandinavia--that reported high rates of concordance for autism among identical twins and no concordance at all among fraternal twins. Last week, the largest and most rigorous twin study ever conducted, the California Autism Twin Study (CATS) reported contradictory new evidence that struck a devastating blow to these claims. The CATS identical twins had lower and the fraternal twins higher concordance rates than past studies, a striking finding that suggests that instead of being highly heritable, the vast majority of autism cases stem from environmental causes.
It’s hard to overstate the importance of the CATS findings. They mean that everything leading “autism experts” have been saying for decades is wrong. And everything leading autism parent advocates have been saying for years is right.
The foundations of autism orthodoxy
As the reality of the autism epidemic began settling in over the last decade, an odd drumbeat in the writing of autism geneticists became more insistent. The more obvious it had become that something new and terrible was happening to a generation of children, the more extreme the statements of the genetics researchers became. It’s as if repeating the orthodox statements as frequently as possible would give them more weight. And the more their extreme claims went unchallenged, the more a spurious “scientific consensus” could be claimed. Here’s a small sample: (1) “Autism is clearly among the most heritable of all psychiatric disorders“(2003); (2) “Autism is one of the most heritable complex disorders, with compelling evidence for genetic factors and little or no support for environmental influence” (2004); (3) “Autism spectrum disorders are considered to be among the most heritable of all mental disorders…. recent reviews estimate the heritability of autistic disorder to be more than 90%.” (2010); (4) “ASDs are known to be highly heritable (~90%)” (2010).
Despite the fact that an explosion in autism rates rendered illogical any ongoing belief in the genetic inheritance of autism, the influence of this orthodox position on autism’s research funding remained profound. Hundreds of millions of research dollars were spent over the last decade in a vain hunt for autism genes; a spending binge that has continued unabated, with over $100 million spent at NIH on genetics-only research during the latest two years of the Combating Autism Act alone.
In the meantime, a mini-industry of epidemic denial has emerged among academics willing to posit a newfound popularity among physicians and parents for the autism label, one that has produced increasingly bizarre claims of diagnostic excess in many forms: substitution, oversight, expansion and accretion among them. These claims have been retracted, disavowed and falsified multiple times, yet because of the overriding need to feed “the hungry lie” these claims keep cropping up in novel forms. Meanwhile, the research funding to support them continues, including active support from NIH. And sadly, anyone in the scientific community with the courage to stick their necks out and suggest that autism rates might be going up because, well, there were more cases of autism, found themselves facing reactions ranging from polite ostracism to ruthless censorship.
The basis for this orthodox belief in autism’s heritability rests on a very specific body of autism research: the investigation of concordance rates within twin pairs. These studies take advantage of a seemingly simple test--the presence or absence of similar outcomes in twins--to estimate the relative contribution of nature (i.e., genes) and nurture (i.e., the environment) to a given disease. To the extent that identical twins have the identical outcomes and fraternal twins have different ones, the cause can reasonably be assigned to genes. To the extent that identical twins have different outcomes and fraternal twins have the same outcome, the cause lies in the environment.
In the case of autism, this test has been applied to twins with increasing frequency, but the orthodox belief in heritability hangs on a slender thread of evidence: the first, a British study of twin pairs first recruited in 1977 (5) and then expanded in 1995 (6); the second, a Scandinavian study from 1989 (7). The 1977 British study reported on just 21 twin pairs, 11 identical and 10 fraternal. Of these, only 4 of the identical twin pairs were concordant for autism (a remarkably low rate that is frequently forgotten). The 1989 Scandinavian study with the same small sample size, 11 identical pairs and 10 fraternal pairs, found autism in both identical twins in 10 of 11 pairs and again no concordance in any of the fraternal pairs. Subsequently, nearly two decades after their first study, the British team recruited 28 new pairs in 1995 and pooled them together with some of the previous group. These new pairs showed a similar profile: only one twin in each of new fraternal pairs had autism (i.e., a zero concordance rate); and of the new identical pairs, 11 of 16 were concordant, giving a 69% concordance rate that fell between their original calculation and the Scandinavian group.
The British team then added an important new element: a heritability calculation. Using a complex approach (with formulae that defy clear explanation), they took all these concordance rates banged them against two different background rates of autism, ran them through a model and declared, “The estimates of broad heritability were 93% and 91% for the base rates of 1.75 and 10 in 10,000.”
And thus was born the belief that autism was more than 90% genetic.
The orthodoxy in quiet crisis: discordant evidence on twin concordance