In science and philosophy, a just-so story, also called an ad hoc fallacy, is an unverifiable and unfalsifiable narrative explanation for a cultural practice, a biological trait, or behavior of humans or other animals. The pejorative nature of the expression 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).
This 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.
This phrase has been used to criticize evolutionary explanations of traits that have been proposed to be adaptations, particularly in the evolution–creation debates and in debates regarding research methods in sociobiology and evolutionary psychology.
However, academics such as David Barash state the term "just so story" when applied to a proposed evolutionary adaptation is simply a derogatory term for a hypothesis. Hypotheses, by definition, require further empirical assessment, and are a part of normal science. Similarly, Robert Kurzban suggested that "The goal should not be to expel stories from science, but rather to identify the stories that are also good explanations."  In his book The Triumph of Sociobiology, John Alcock suggested that the term just so story as applied to proposed evolved adaptations is "one of the most successful derogatory labels ever invented".
- David J. Buller (2005). Adapting Minds: Evolutionary Psychology and the Persistent Quest for Human Nature. MIT Press. pp. 86–87. ISBN 978-0-262-02579-9.
- Glen A. Love (2003). Practical Ecocriticism: Literature, Biology, and the Environment. University of Virginia Press. p. 71. ISBN 978-0-8139-2245-4.
- Rudyard Kipling (1902). "How the Leopard got his Spots". Just So Stories for Little Children. Retrieved 2013-4-13.
- Mark Isaak (2007). The Counter-Creationism Handbook. University of California Press. p. 102. ISBN 978-0-520-24926-4.
- David Barash (September 17, 2012). "An Unevolved Take on Psychology". The Chronicle of Higher Education. Retrieved 2013-4-13.
- Robert Kurzban (2012). "Just So Stories Are (Bad) Explanations. Functions Are Much Better Explanations". Evolutionary Psychology. Retrieved 2013-4-13.