copyright © 2002, Nathaniel Lane Taylor
The legend of Pope Joan, a female claricwho passed for male during a long career which included over two years as pope—until she gave birth in a public procession—seems to have originated in the early fourteenth century. It was a recurring legend in the late Middle Ages and Renaissance, first found among Catholic writers (not all of whom used it as an anti-clerical trope) and then among Protestant anti-Catholics, who adopted her as an example of the moral corruption and invalidity of the Papacy.
Two recent studies take different stances on the legend's veracity. Peter Stanford's Pope Joan (Cambridge, 2001)  is a credulous 'expos' which did not deserve the publicity it received when it was being promoted in the US. A cultural history of the legend by Alain Boureau (1988; published in English in 2001) tells the much more interesting story of the genesis and misappropriation of an obscure and certainly fictitious story.  This supercedes an older work in English by Rosemary and Darryl Pardoe, which is methodical and useful in listing and summarizing the sources and evolution of the legend. The German work on Joan is also superior to the credulous Stanford book. 
One theory suggests that the story might have originated with, or been conflated with, the story of Marozia, a woman of a leading Roman aristocratic family (the Theophylacts) who was the mistress, mother and ancestress of several popes in the tenth century. She is mentioned by Liudprand, Antapodosis 2:48, 3:46). Among the many modern literary (or sub-literary) fugues on Pope Joan, I have enjoyed the obscure trashy novel (essentially a bodice-ripper) by E. L. Hastings (a pseudonym?), Pope Joan (Wayne, Pennsylvania, 1984), which is based on the story of Marozia.
There are also many modern novelizations of the Pope Joan theme.
 Peter Stanford, The she-pope: a quest for the truth behind the mystery of Pope Joan (London: Heinemann, 1998); subs. pub. in USA as The legend of Pope Joan: in search of the truth (Holt, 1999).
 Alain Boureau, The myth of Pope Joan, trans. Lydia G. Cochrane (Chicago, 2001).
 Rosemary Anne Pardoe and Darroll Pardoe, The female pope: the mystery of Pope Joan: the first complete documentation of the facts behind the legend (Wellingborough, Northants., 1988). Elisabeth Gössmann, Mulier Papa: der Skandal eines weiblichen Papstes: zur Rezeptionsgeschichte der Gestalt der Päpstin Johanna (Munich, 1994). See also Päpstin Johanna: ein Lesebuch, ed. Klaus Völker (Berlin, 1977).