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.

Examples:

I Believe In Miracles

Damn, now that song is going through my head. Who was that? Butch says it was Wild Cherry, but don't hold me to it. Anyway, my friend Eric sent me a link to Michael Shermer's site, to a page titled Miracle on Probability Street. He wrote it in 2004 but I thought I'd share it with you because it's very good information.

We've all experienced a highly improbable event in our lives. Probably many, in fact. Some of us more than others, some more seemingly improbable than others. There is such a thing as the Law of Large Numbers that explains these coincidences and "miracles".

The Law of Large Numbers simply stated (sans math): with a large enough sample many odd coincidences are likely to happen.

Coincidence: an occasion when two or more similar things happen at the same time, especially in a way that is unlikely and surprising.

Miracle: an unusual and mysterious event that is thought to have been caused by a god, or any very surprising and unexpected event.

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On a side note, I was disappointed with Dictionary.com's listing on these words so I thought I'd go to the Cambridge Dictionary. The definition above is from the Dictionary of British English. Out of curiosity, I looked up the word miracle in the Cambridge Dictionary of American English:

Miracle: an unusual and mysterious event that is thought to have been caused by God, or any surprising and unexpected event.

A very subtle but telling difference! I think I'll be using the British version from now on.