A just-so story, also called the ad hoc fallacy, is a term used in academic anthropology, biological sciences, and social sciences. It describes an unverifiable and unfalsifiable narrative explanation for a cultural practice or a biological trait or behavior of humans or other animals. The use of the term is an implicit criticism that reminds the hearer of the essentially fictional and unprovable nature of such an explanation. Such tales are common in folklore and mythology (where they are known as etiological myths — see etiology).

The phrase was popularized by the publication in 1902 of Rudyard Kipling's Just So Stories, containing fictional and deliberately fanciful tales for children, in which the stories pretend to explain animal characteristics, such as the origin of the spots on the leopard (e.g., "How the Leopard Got His Spots").

See also

• Texas sharpshooter fallacy
• Is-ought problem