The ad populum/appeal to popularity fallacy can be be though of as a subspecies of argument from illegitimate authority. However, we should be careful because sometimes what a crowd wants is a legitimate reason to believe the claim.

Here  are some different cases:

Illegitimate Ad Populum

A: Big Pharma is hiding the cure to cancer.
B: Really? How do you know?
A: Just look at the latest Gallup Polls. 65% of Americans believe this to be true.

Legitimate Ad Populum
When something’s being true depends on the attitude of a group. E.g., being in fashion, being popular, being cool, etc…

A: Bellbottoms are cool.
B: Really? How do you know?
A: Cuz in a recent survey, 80% of Americans thought so.

2. When the group’s attitude being measured is a group of relevant experts.

A: Anthropogenic climate change is real.
B: That’s not what I heard. Why do you believe that?
A: Well, over 90% of climate scientists agree.

Examples

1. (This one combines two fallacies)

 

Answers and Notes

1. Ad Populum with Tu Quo Que