William Reville: The likely cause of the ‘rise’ in autism

Autism is a neurodevelopmental disorder characterised by impaired social interaction, impaired verbal and nonverbal communication, and restricted, repetitive behaviour. In the early 1990s the diagnosis of autism broadened to cover autism spectrum disorder (ASD), characterised by a broad range of these symptoms.

The causes of ASD are under investigation and are a source of intense popular interest. The case for some proposed causes of ASD, for example vaccines and ingestion of certain herbicides, are often significantly informed by pseudoscience.

The incidence of ASD is widely reported to be on the increase: the US Centres for Disease Control and Prevention reported a 30 per cent increase in the past few years alone. But is the reported increased incidence of ASD real or only an apparent increase largely explained by new reporting practices? Two new studies support the latter interpretation.

Heritability of autism has been studied, and a genetic cause can be elucidated in 10-30 per cent of ASD cases. Many studies have investigated non-genetic causes of ASD. Among environmental risk factors that have been associated with ASD are parental age, prenatal and perinatal complications, air pollution and maternal exposure to pesticides.

Advocates of alternative medicine and natural products have particularly focused on a link with glyphosate, the active ingredient in the weedkiller Roundup. Roundup is used on genetically modified crops that resist this herbicide. However, as far as I am aware, no credible research and no data link glyphosate to autism. Despite the lack of evidence, one prominent campaigner claims that by 2025, one in two children (in the US) will be autistic largely due to glyphosate. Yet the few studies that have specifically investigated a possible link between glyphosate and ASD have failed to find the link, and there is no positive correlation between clusters of ASD in the US and the use of Roundup.

Correlation versus causation

The case linking glyphosate with ASD is largely based on correlation; that is, as the use of glyphosate increased, the incidence of ASD increased over the same time. This argument ignores the elementary fact that correlation does not imply causation. There is no shortage of items, for example electronic gadgets, whose use increased over the same time that ASD incidence has increased. For example, the sale of organic foods rose exactly in parallel with increasing numbers diagnosed with ASD over the past 20 years, but the anti-GM people are silent about this “cause” of ASD.

The 1998 study by Andrew Wakefield and others, published in the Lancet, that linked autism to the MMR vaccine has been comprehensively debunked by now, but the study scared many parents into not vaccinating their children. Unfortunately, the legacy of the Wakefield affair persists, stoked by activists who ignore the mountain of evidence that vaccination is safe under all sensible definitions of this term.

The reclassification of autism to ASD in the early 1990s ushered in an expanding list of symptoms and a more subtle classification of autistic behaviours. More types of behaviour profiles are now classified as autistic, raising the possibility that this broadening alone explains the increasing diagnoses of ASD.

There is now data to support this notion. The most recent study, from Sweden, was published online by Sebastian Lundstrom and others in the British Medical Journal last month, and a similar study from Denmark was published in January 2015by Stefan Hansen and others in JAMA Pediatrics.

The Swedish study found that over a 10-year period (1993 to 2002), although the number of people diagnosed with ASD increased, the numbers with actual autism phenotypes remained stable. The Danish study found that more than 60 per cent of the increase in ASD diagnoses can be attributed to reporting practices, not to an increase in patients with ASD.

Calling the increased diagnoses of ASD an epidemic prompts people to think that there is a single underlying environmental cause, just as malaria epidemics are caused by a parasite carried by mosquitoes. If you identify the environmental cause, you can eliminate the epidemic.

But it now seems that most of the apparent increase in the incidence of ASD is down to reporting practices. It is time to refocus efforts to find the causes of ASD, concentrating less on environmental causes and more on genetics.

William Reville is an emeritus professor of biochemistry at UCC, understandingscience.ucc.ie