Accused of a fallacy? Suspect a fallacy? Ask Dr. Bo and the community!

Quickly register to comment, ask and respond to questions, and get FREE access to our passive online course on cognitive biases!

one moment please...

Disjunction Fallacy

Description: Reasoning that it is more likely that a member is part of a subset rather than a member of the set which contains the subset.  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 alternative.

Logical Forms:

Event X is more likely than event X or Y.


Event Y is more likely than event X or Y.

Example #1:

Mr. Pius goes to church every Sunday.  He gets most of his information about religion from church and does not really read the Bible too much.  Mr. Pius has a figurine of St. Mary at home.  Last year, when he went to Rome, he toured the Vatican.  From this information, Mr. Pius is more likely to be Catholic than a Catholic or a Muslim.

Explanation: This is incorrect.  While it is very likely that Mr. Pius is Catholic based on the information, it is more likely that he is Catholic or Mulsim.

Example #2:

Bill is 6’11” tall, thin, but muscular.  We know he either is a pro basketball player or a jockey.  We conclude that it is more probable that he is a pro basketball player than a pro basketball player or a jockey.

Explanation: This is incorrect.  While it is very likely that Bill plays the B-ball, given that he would probably crush a horse, it is statistically more likely that he is either a pro basketball player or a jockey since that option includes the option of him being just a pro basketball player.  Don’t let what seems like common sense fool you.

Exception: None.


Gilovich, T., Griffin, D., & Kahneman, D. (2002). Heuristics and Biases: The Psychology of Intuitive Judgment. Cambridge University Press.

Registered User Comments

Become a Logical Fallacy Master. Choose Your Poison.

Logically Fallacious is one of the most comprehensive collections of logical fallacies with all original examples and easy to understand descriptions; perfect for educators, debaters, or anyone who wants to improve his or her reasoning skills.

Get the book, Logically Fallacious by Bo Bennett, PhD by selecting one of the following options:

Not Much of a Reader? No Problem!

Enroll in the Mastering Logical Fallacies Online Course. Over 10 hours of video and interactive learning. Go beyond the book!

Enroll in the Fallacy-A-Day Passive Course. Sit back and learn fallacies the easy way—in just a few minutes per day, via e-mail delivery.

Have a podcast or know someone who does? Putting on a conference? Dr. Bennett is available for interviews and public speaking events. Contact him directly here.

About Archieboy Holdings, LLC. Privacy Policy Other Books Written by Bo
 Website Software Copyright 2019, Archieboy Holdings, LLC.