Logical Fallacy 12: The Availability Heuristic

A recent memory, an unusual event, or a striking anecdote leads us to overestimate the probability of events of that type occurring. (this is also called the Anecdotal Fallacy)

This is Part 12 in a series about Logical Fallacies. We are going through one fallacy at a time. There are many types of fallacious arguments. I’m going to try to explain them with examples then find ways to help you refute those arguments when they occur. Please comment or email if there’s a particular fallacy you want me to tackle, or if you have success with refuting an argument using a good technique you can share.

First, let's define the terms:

Heuristic: experience-based techniques that help in problem solving, learning and discovery. A heuristic method is used to come to a solution rapidly that is hoped to be close to the best possible answer, or 'optimal solution'. A heuristic is a "rule of thumb", an educated guess, an intuitive judgment or simply common sense. A heuristic is a general way of solving a problem.

Heuristics are basically evolutionarily good. They simplify and speed up decision making, using less energy to find a decent solution, especially when you don't really have enough information to make an educated decision, for when things are uncertain. For example, you have seen 2 people mauled by lions. It's a safe bet that if you run into a lion it will maul you. You don't need to actually be mauled by a lion yourself. When you go out to hunt, you will therefore do something to protect yourself from being mauled. Unfortunately heuristics are often irrational and instinctive.

Availability Heuristic: determining probability by the ease with which relevant examples come to mind, or by the first thing that comes to mind.

  • You see an accident while driving. You then slow down and drive more carefully for a little while thinking it could happen to you. Your chances of getting in an accident haven't changed just because you saw one and it was fresh and vivid in your mind.
  • Lotteries don't talk about the odds of you winning to sell you a ticket. They talk about who won recently. The news helps with the way they report who wins, focusing on anything unusual or emotional about who won. A person is more likely to buy a ticket if the first thing that comes to mind is winning rather than losing. (You've probably got a better chance of being killed in a car accident driving to buy your lottery ticket than you do of winning the lottery.)
  • Where an anecdote ("I know a Brazilian man who...") is used to "prove" an entire proposition or to support a bias, the availability heuristic is in play. In these instances the ease of imagining an example or the vividness and emotional impact of that example becomes more credible than actual statistical probability. Because an example is easily brought to mind or mentally "available," the single example is considered as representative of the whole rather than as just a single example in a range of data.
  • A person claims to a group of friends that drivers of red cars get more speeding tickets. The group agrees with the statement because a member of the group, "Jim," drives a red car and frequently gets speeding tickets. The reality could be that Jim just drives fast and would get a speeding ticket regardless of the color of car that he drove. Even if statistics show fewer speeding tickets were given to red cars than to other colors of cars, Jim is an available example which makes the statement seem more plausible. This is an example of using anecdotal evidence to "prove" a point. While it gives the impression of legitimacy, it's invalid.

How to Refute the Availability Heuristic

This is a tough one. We all use heuristics and they seem very natural, and seem to make sense. I myself am afraid to fly when I know intellectually that flying is much safer than driving a car. I think the only way to refute this logical fallacy is to explain anecdotal evidence (and how invalid it is), and to explain how heuristic thinking works, followed by more statistics that show the real probability of a given event, if you have them available.


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