Autism Caused by Vaccines?
Autism-vaccine controversy has lately captured (more of) my attention. You can’t search for “autism” or “vaccine” even separately on Google without stumbling across a dozen blogs dedicated to this subject¹. This is obviously an area of  high passions. People’s children are being diagnosed with very scary, fundamentally unsettling disorders at increasingly high rates (although the rate of increase of the rate seems to be slowing down, a fact cited as support by both sides of the debate), and nobody has any solid answers about what causes this, or how to fix it.

Though of considerably lesser consequence, epidemiologists’ competence is also being called into question. I’m sure this has at least something to do with the drama.

It seems  that the “vaccines cause autism” side of things is being driven largely by reports of people whose children have received vaccines and then been diagnosed with autism, countered by a boatload of empirical studies suggesting the there is no link. Personally, I think the studies so far still leave room for a possible association (perhaps only for a few individuals?), but this would not be the broad-strokes disease model suggested by the so-called “mercury militia.” If vaccines imparted a general, across-the-board risk for autism — even a fairly small or inconsistent one — it would almost certainly have shown up in spades by now. It hasn’t.

The Null Hypothesis
Whether vaccines cause autism or not, the discussion illustrates a sinister mental error we humans make. Well, more than one, actually². The null hypothesis can help illustrate the concepts here³. The null hypothesis is a fairly simple and logical concept, yet we so rarely apply it. It is simply the question,

“What would I expect to see if my suspicions weren’t correct?”

See how insidious that is? Nobody likes to sit around and seriously consider the possibility that they may be wrong⁴. So, let’s look at this possibility, as much as a lazy sometime-blogger can, within a limited amount of time and not being paid to do this.

In case this is not clear by now, I think people may be tempted to assume (especially if they never stop to think about it carefully) that a lack of association between vaccines and autism would mean that nobody ever had the experience of autism being diagnosed fairly soon after a vaccine. However, I will show that this is not the case at all. In fact, if there’s no association, we should still expect to see quite a number of cases like that.
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1. The blogs tend to be written by nonscientists interested in asserting that vaccines cause autism, whereas the scientists don’t usually have blogs arguing back [↩]
2. Repeat after me: correlation cannot prove causation [↩]
3. Despite its sullied reputation in peer-reviewed research [↩]
4. Did I mention the scientific method? It can help with this kind of stuff [↩]