Using the opinion or position of an authority figure, or institution of authority, in place of an actual argument.
Appeals to authority are by far the most commonly used means of advertising products, whether religious, conspiratorial, all the way down to hair care products. Open up a magazine and you’ll see some famous person, likely with some letters after their name, approving of whatever is being sold, whether or not they have any expertise in that specific area.
4 out of 5 dentists recommend this toothpaste!
Note the above quote is not a fallacy, as dentists are highly likely to have knowledge as to which toothpastes are good for you and your teeth and which aren’t. Given the lack of a fallacy, the next question should be, what would cause 20% of dentists to not recommend that toothpaste?
The image to the right shows Marilyn Monroe giving her a-okay to some shampoo. How many years of chemical engineering and dermatology study do you think she has on her resume? I’m going to guess very little to none, which makes her promoting this product a fallacy.