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Cognitive Biases Make Reasoning About Vaccines Difficult

How to watch out for the base rate fallacy.

Key points

  • Cognitive biases make reasoning about vaccines difficult.
  • The base rate fallacy is a key cognitive bias.
  • You can learn to watch out for the base rate fallacy.

When it comes to thinking about the likelihood of events in the world, humans are woefully lacking. Indeed, when we reason about likelihoods and probabilities, we’re susceptible to all manner of mistakes. Psychologists call these susceptibilities cognitive biases. Cognitive biases can lead to faulty thinking and bad decision-making, sometimes with life-and-death consequences.

One type of cognitive bias at play these days on social media, and even in traditional media, has to do with the likelihood of becoming ill from Covid-19 following vaccination. Psychologists call the source of this bias the base rate fallacy.

To make the base rate fallacy concrete, consider the following situation:

Let's suppose you hear that following a party attended by ten people, one vaccinated person and one unvaccinated person became ill from Covid-19. You might think, "Wow, one out of two people (50 percent) who got Covid were vaccinated! So why bother to get vaccinated?"

This is basically the argument you see every day all over social media. But what’s wrong with this way of thinking?

To see the problem, imagine I tell you that at this same party with the two Covid-19 illnesses, both people who became ill were right-handed. Would you then conclude that if you’re left-handed, you’re safe? Probably not. But why not? The answer, of course, is that because there are far more right-handed people in general (90 percent of us are right-handed), there will also be far more right-handed illnesses. In other words, the chances that a person who becomes ill is right-handed is already much higher simply because right-handedness is much more common.

The important point here is the idea that the overall prevalence of something in the population, like right-handedness versus left-handedness, matters when thinking about how likely something is in those two groups. This overall prevalence is the base rate, and our tendency to ignore the overall prevalence is the base rate fallacy. The base rate of being right-handed is very high, so there will be more of just about anything in right- than left-handers, including illness from Covid-19, simply because of this difference in base rate. More right-handers than left-handers eat hamburgers every day. But that doesn’t mean right-handers like hamburgers more than left-handers. It just means there are more right-handers.

Now if we replace “right-handed” with “vaccinated” we can start to see why the base rate fallacy matters in any vaccination discussion. Just like with right-handedness, as vaccination rate increases and there are more vaccinated and fewer unvaccinated people in the population, the absolute numbers of vaccinated people who become ill will increase.

Suppose you now learn that at this party with two Covid-19 illnesses, out of ten people, eight people were vaccinated and two were not. You might still be tempted to say that 1 out of 2 ill people were vaccinated, but that clearly misses the important point about base rates. The right way to think about this is that the one vaccinated person who became ill was one out of eight vaccinated people (12.5 percent), but the one unvaccinated person who became ill was one out of two unvaccinated people (50 percent). Very different, right? The conclusion you’d reach from this example when the base rate is considered is that you would have been about four times more likely to become ill at this party if you’d been unvaccinated. In fact, the real numbers are even more strongly tilted in favor of vaccination. And we're all at this party.

It’s not surprising that most of us have a tough time thinking about these situations. Our brain is just wired this way, with cognitive biases built in. Luckily, becoming aware of these biases can help us to spot them when they arise, and ultimately to avoid them. Remember, when thinking about how likely something is in different groups, base rate matters.

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