What Is a Conjunction Fallacy?

The conjunction fallacy occurs when we mistakenly believe specific conditions are more probable than a single general one. It's a mental shortcut that often leads us astray, particularly in decision-making. This cognitive error reveals the quirks of human reasoning. Have you ever wondered how this fallacy might be influencing your choices? Let's examine its impact on our everyday judgments.
G. Wiesen
G. Wiesen

A conjunction fallacy is a type of probability fallacy in which people, when offered the choice between one event and that event plus another event, are more likely to choose the second option as more probable. In other words, some people presented with the question “What is more likely to occur: a dog running across a street, or a dog running across a street and barking at a cat” tend to choose the second option. A conjunction fallacy occurs because people often do not consider that for a conjunction to be true, each part of it must be true, and because options with greater quantity are somehow more attractive.

The basic concept behind the conjunction fallacy is the way in which people tend to view two similar options. In this instance, the options are two types of events or situations in which one is part of the other. An example of how this fallacy can occur would be the following statement: “A man wakes up every day at six in the morning. When he wakes up, is he more likely to drink coffee, or to drink coffee and then brush his teeth?” In answer to this question, people often have the tendency to choose the more complex answer and commit a conjunction fallacy.

Woman holding a book
Woman holding a book

The key to recognizing the conjunction fallacy is in understanding and knowing how to identify it. Statistically speaking, a conjunction must be considered as two separate parts, such as “the man drinking coffee” and “the man brushing his teeth.” This means that the second option has two elements that must both be true for it to be more probable than the other option, which only has one element that must be true. Since the second option in a conjunction fallacy contains the first, it is easier for the first option to be true, as it does not rely on a second possible element.

This means that the simpler option is more probable, by the very nature of the argument. Even people aware of the statistical reality behind the conjunction fallacy can easily fall into it, due to the fact that it seems to be innately more appealing. For some reason, people seem to prefer an option that is more complex or seems more developed and decide that it is a more likely or probable situation. This is why someone must understand and know how to recognize the conjunction fallacy to avoid it, as mathematical or statistical background may not be sufficient.

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Discussion Comments


@Euroxati - Here's yet another example - A man is bored and wants to watch the Amazing Spider-Man 2. When he goes to the movie theater, is more likely to only buy his ticket, or buy popcorn and snacks as well?

Whether it's "right" or not, most people would choose the second answer. Not only is it more complex, but most people (if not everyone) buy a snack of some sort (whether it's popcorn or candy) when going to the movie theater.


Does anyone have another example of a conjunction fallacy? While the article did give a few good examples, I would like to see another one.


In my opinion, fallacies can definitely be tricky. Not only can people tend to be really clever with them, but when it all comes down to it, it actually makes a lot of sense, as the article indirectly points out. Also, in this case, I think the reason why people choose the second answer is not just because it's more complex, but it has more meaning behind it.

Referring to the example the article gave, drinking coffee and brushing your teeth would make a lot more sense, because that's what most adults do when they get up in the morning, instead of just drinking coffee. Interesting article.

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