One common feature of unscientific belief systems is that they do not change in the face of new evidence. They tend to evolve like cultural beliefs or marketing campaigns, but do not appear to be affected by scientific evidence in any meaningful way.

One great example of this is the idea the autism is linked to vaccines (to be clear up front, it isn’t) This idea had a few important factors in its origin. The first was simply the existing anti-vaccine movement searching for anything to blame on vaccines. The second, and perhaps decisive, factor was the now discredited and withdrawn study by Andrew Wakefield linking autism to the MMR vaccine.

Even as the MMR claim was dying, the anti-vaccine community was moving onto the next target – mercury (specifically the preservative Thimerosal). This was the target of the book Evidence of Harm by David Kirby. This also created common cause between the anti-vaccine movement, and separate “mercury militia” blaming many modern ills on mercury, and some environmentalists (most prominently Robert Kennedy Jr.) who are keen to blame medical problems on any environmental exposure, including mercury and/or vaccines.

There are several lines of evidence that bear on the question of whether or not mercury or any vaccine exposure contributes to the incidence of autism. We can simply ask observationally is there any correlation between any type of vaccine exposure and the risk of developing autism. After years of study the short answer is no – there is no demonstrable link between vaccines and autism.

Because mercury and thimerosal as a vaccine ingredient is often blamed, we can say separately that there is no correlation between exposure to thimerosal and autism. In fact, thimerosal was removed from the routine vaccine schedule in the US around 2002. At the time antivaxxers (including David Kirby) predicted that autism rates would plummet – they didn’t. They have continued to rise at the exact same rate without a blip. (This is likely to change eventually as the diagnosed rate reaches the true rate, and also because of changing diagnostic criteria.)

We can also explore the causes of autism. If we find that autism, for example, is primarily a genetic disorder, then there may not be a significant role for environmental factors. In fact the research has been showing that autism is a dominantly genetic group of disorders (likely not one discrete entity). The anti-vaccine community mostly ignores this evidence, or dismisses it with a wave of the hand but without mounting even a single coherent argument that I have ever heard.

Part of what leads some parents to believe that vaccines caused their children’s autism is that the clinical signs of autism become apparent after children have received many of their vaccines. The childhood vaccine schedule includes many shots between birth and 18 months. While age of diagnosis can vary, many parents notice the signs of autism between 1 and 3 years of age and so there is likely to be some apparent temporal association by chance alone.

This raises another important question in the vaccine – autism controversy – at what age does autism truly begin? Careful scientific studies have found clinical signs of autism as early as 6 months of age. This is before the first MMR dose is typically given, which is at 12 months. If the earliest observable signs of autism are present at 6 months it is very likely that the processes changing brain structure and function begin before six months. The true onset of autism is therefore before all but the earliest vaccine doses.

This makes it much more difficult to blame autism on vaccines. This, of course, doesn’t stop from doing so.

A new study sheds further light on this question and likely pushes back the onset of autism into the womb. The researchers examined the brains of children with autism and typical children who happened to die. They found that the brains of most of the autistic children contained patches of disorganized neurons (especially in layers 4 and 5 and the prefontal cortex, for those interested). The organization of neurons occurs primarily in the third trimester. The most plausible explanation for these findings is that they are resulting from a process that affects brain organization at this developmental stage – in the womb.

Of course, this is just one exploratory study. Like all new findings in science, they need to be replicated. The results are fairly objective and robust, however.

Assuming the results pan out, this would provide further evidence that autism is likely to be a genetic disorder affecting early brain development. Onset in the womb, of course, would eliminate childhood vaccines as a cause.

We already know, however, that this won’t stop the anti-vaccine community from continuing to claim that vaccines or mercury cause autism. The evidence is not relevant to their beliefs. They already have shifted to blaming the mother’s vaccines.

Conclusion

Multiple independent lines of evidence strongly suggest that autism is a dominantly genetic disorder affecting early pre-natal brain development. Further, there is no link between vaccines or mercury and the risk of developing autism. This new study is just one tiny piece of the overall puzzle, but it is consistent with the big picture.

I have yet to hear of any prominent anti-vaccine activist, however, alter their rhetoric about vaccines in response to scientific evidence. Perhaps there is an exception out there I have not heard of, but it would be just that – an exception. For the most part the anti-vaccine movement has not budged from their position, despite the scientific evidence.

Like other science-denial and anti-science movements, they have their studies to cherry pick, their fringe researchers to idolize (like Andrew Wakefield and the Geiers), and their conspiracy theories to dismiss all inconvenient evidence.