While studying informal fallacies to prepare the next lesson in my upcoming rhetoric text Fitting Words, I observed something surprising about the ad populum fallacy. I have generally understood this fallacy to be simply an appeal to the masses, indeed to the mere mass of the masses” (Introductory Logic, Nance and Wilson) as in “Mom, all my friends are doing it!” (for which all mothers have learned the counterexample “If all your friends were jumping off of a cliff, should you?”)

But in all the standard college logic texts that I own (not a small sample), argumentum ad populum is taken to mean, quite broadly, an improper appeal to emotion. As a sample:

• Argumentum ad populum, see appeal to emotion” – Kelley, The Art of Reasoning. (Kelley includes a separate fallacy called “Appeal to Majority.”)
• Ad populum (appeal to emotion): An informal fallacy in which the support given for some conclusion is an inappropriate appeal to popular belief, or to the emotions of the audience.” – Copi, Introduction to Logic
• Ad populum: An informal fallacy that occurs when an arguer plays on certain psychological needs for the purpose of getting the listener to accept a conclusion.” – Hurley, A Concise Introduction to Logic

But googling ad populum on the Internet returns these top definitions:

• “In argumentation theory, an argumentum ad populum (Latin for ‘appeal to the people’) is a fallacious argument that concludes that a proposition is true because many or most people believe it: ‘If many believe so, it is so.’ ” – Wikipedia
• “The ad populum fallacy is the appeal to the popularity of a claim as a reason for accepting it.” – The Skeptic’s Dictionary
• “Appeal to Popularity (also known as: Ad Populum). The Appeal to Popularity has the following form: Most people approve of X (have favorable emotions towards X). Therefore X is true.” – The Nizkor Project
• “Argumentum ad populum: Appeal to common belief. Description: When the claim that most or many people in general or of a particular group accept a belief as true is presented as evidence for the claim. Logical Form: A lot of people believe X. Therefore, X must be true.” – http://www.logicallyfallacious.com

These are two very different views of this fallacy: 1)  appeal to emotion; 2) appeal to the masses. To be sure, with some creative wording these two meanings of ad populum can be somewhat reconciled, but on the face of it a clear mitosis has taken place somewhere between the college textbooks and social media.

Things that make you go “Hmm.”