In science and philosophy, a just-so story is an untestable 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 listener of the essentially fictional and unprovable nature of such an explanation. Such tales are common in folklore genres like mythology (where they are known as etiological myths – see etiology). A less pejorative term is a pourquoi story, which has been used to describe usually more mythological or otherwise traditional examples of this genre, aimed at children.

This phrase is a reference to Rudyard Kipling's 1902 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. It 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, the first widely acknowledged use of the phrase in the modern and pejorative sense seems to have originated in 1978 with Stephen Jay Gould, a prominent paleontologist and popular science writer. Gould expressed deep skepticism as to whether evolutionary psychology could ever provide objective explanations for human behavior, even in principle; additionally, even if it were possible to do so, Gould did not think that it could be proven in a properly scientific way.