Seen on Facebook: “Part of the reason we have diseases is because of the shit they put in the vaccinations.”

Then they argued against vaccinations. The usual followed: That vaccines cause autism (a scientifically discredited idea) and that ‘other chemicals’ in the vaccines cause disease by ‘attacking our organs’ (actually, such chemicals are in significantly lower concentrations in vaccines than in most foods and drinks). They then ‘agreed to disagree’, demonstrating no flexibility on the issue. It is terribly frustrating given the important benefits of a major public health measure.

What is going on here? Why do people believe the wrong story in the face of the overwhelming scientific evidence?

The answer is not that they are stupid. It is likely a result of pernicious cognitive biases that we all share. We’re all pretty hopeless at assessing small risks and our brain is essentially useless when it comes to statistics. It’s not natural to think statistically and even statisticians don’t do it automatically. Most importantly here, we tend to construct arguments in our minds based on the easiest memories we can access. This is called “availability bias”, bolstered by its cousin, the salience of vivid events creating an overestimation of the probabilities.

We fixate on rare dramatic events, and they become etched into our memories with all the emotional adornments of terror and compassion. They are easy to recall, and we then construct stories and causation from them. A child gets very sick after a vaccination, we are horrified, we link that to a well publicised (but wrong) link between autism and vaccinations and hey presto! Vaccines are to blame. It seems to make sense and gives us warm feelings that we have understood the world and can proceed with that knowledge.

None of that process requires you to think hard. You do it almost automatically. In contrast, to actually examine the link requires deliberate thought. Any medical treatment carries risk, but it is usually exceptionally small, and mostly well worth discounting. We attach too much value to very slim chances. For instance, a small risk such as a 1 in 10,000 chance of an adverse outcome is, perversely, perceived as hugely worse than no chance at all, and so we tend to avoid the risk by avoiding the risky activity.

On the flipside, we are happy to accept the extremely high probability of losing money in a lottery because we cannot really grasp the vanishingly small chance of winning – but we have no trouble at all imagining the benefits! Don’t trust your gut on small probabilities.

Autism is not caused by vaccinations, but that fact is far less famous than the original headliner. So powerful is the availability bias, that it affects the news and this creates a cascade of availability, reinforcing your biases. People ask, ‘but what if it is true, shouldn’t we be cautious?’ That is your little availability machine speaking to you. Fight back, question whether the information you’re using is true. Extreme caution is warranted only in the face of true uncertainty; it should not be a default position. It hardly needs to be said that not vaccinating causes a significantly higher risk of disease spreading in society.

Your fast-thinking brain is very good at living, but it is designed to meet challenges that are simple in nature, like avoiding lions on the plains of Africa. It’s not so good at new complex ideas, but it can be, as long as you work at it. Don’t let your first thoughts pervade your life; instead, use your capacity for reason to enhance it.

This first appeared in print in my column in Woroni, the student newspaper of The Australian National University, 29 August 2013.