search

Conjunction Fallacy

(also known as: conjunction effect)

Description: The assumption that more specific conditions are more probable than general ones.  This fallacy usually stems from thinking the choices are alternatives, rather than members of the same set.  The fallacy is further exacerbated by priming the audience with information leading them to choose the subset as the more probable option.

Logical Form:

X is a subset of Y.

Therefore, X is more probable than Y.

Example #1:

While jogging around the neighborhood, you are more likely to get bitten by someone’s pet dog, than by any member of the canine species.

Explanation: Actually, that is not the case.  “Someone’s pet dog”, assuming a real dog and not some robot dog, would also be a member of the canine species.  Therefore, the canine species includes wolves,  coyotes, as well as your neighbor’s Shih Tzu, who is likely to bite you just because he’s pissed for being so small.

Example #2: Mr. Pipp, is a sharp dresser, too good-looking, works as an interior decorator and loves everything Barbra Streisand.  Is Mr. Pipp more likely to be a man or a gay man?

Explanation: It would be fallacious to say that Mr. Pipp is more likely to be a gay man—even if we found out that Mr. Pipp worked nights as a dancer at a drag queen show.  There is a 100% chance Mr. Pipp is a man, and a smaller chance that he is a gay man because the group “man” includes all the members of the group “gay man”.

Exception: When contradicting conditions are implied, but incorrectly stated.

In the example above, the way the question reads, we now know that there is a 100% chance Mr. Pipp is a man and a smaller chance that he is a gay man.  However, if the questioner meant to imply, “straight man” or “gay man” as the choices, then it could be more of a poorly phrased question than a fallacy.

References:

Kahneman, D. (2013). Thinking, Fast and Slow (1st edition). New York: Farrar, Straus and Giroux.

Questions about this fallacy? Ask our community!

Uncomfortable Ideas: Facts don't care about feelings. Science isn't concerned about sensibilities. And reality couldn't care less about rage.

This is a book about uncomfortable ideas—the reasons we avoid them, the reasons we shouldn’t, and discussion of dozens of examples that might infuriate you, offend you, or at least make you uncomfortable.

Many of our ideas about the world are based more on feelings than facts, sensibilities than science, and rage than reality. We gravitate toward ideas that make us feel comfortable in areas such as religion, politics, philosophy, social justice, love and sex, humanity, and morality. We avoid ideas that make us feel uncomfortable. This avoidance is a largely unconscious process that affects our judgment and gets in the way of our ability to reach rational and reasonable conclusions. By understanding how our mind works in this area, we can start embracing uncomfortable ideas and be better informed, be more understanding of others, and make better decisions in all areas of life.

Get 20% off this book and all Bo's books*. Use the promotion code: websiteusers

* This is for the author's bookstore only. Applies to autographed hardcover, audiobook, and ebook.

Get the Book