Argumentum ad populum are appeals to popularity or appeals to majority. Copi and Cohen (1994) wrote, “it [an argument ad populum] is fallacious because it replaces the laborious task of presenting evidence and rational argument with expressive language and other devices calculate to excite enthusiasm, excitement, anger, or hate.”
This fallacy has a simple and common example, “everybody else is doing it, so it can’t be wrong.” Or as the famous line goes, “one thousand Frenchmen can’t be wrong.”
One very common type of this fallacy is the bandwagon appeal. Where a debater attempts to win over listeners by telling that all the popular or cool people are doing something, so it must be good.
At the heart of every argumentum ad populum lies an appeal to pathos (emotion). Argumentum ad populum appeal to everyone’s deep emotional want to be liked, accepted, or feel as part of a group. However, this is still fallacious. Appeals to emotion though acceptable in common argument are out of place in a debate unless extremely well frame (though even then this is likely to be fallacious).
The fact that many people accept or support an idea does not actually prove the validity of that idea. The philosopher Bertrand Russell said of argumentum ad populum (as quoted in Copi and Cohen, 1994):
The fact that an opinion has been widely held is no evidence whatever that it is not utterly absurd; indeed, in view of the silliness of the majority of mankind, a wide-spread belief is more likely to be foolish than sensible.
Copi, I. M. & Cohen, C. (1994). Introduction to Logic (ed. 9). New York: MacMillan Publishing Company.