Pope Joan is the name of a female pope (also La Papessa) who supposedly reigned for less than two years in the 850s, based on a legend that circulated in the Middle Ages. Pope Joan is regarded by most modern historians and religious scholars as fictitious, possibly originating as an anti-papal satire.

The legend of Pope Joan
In 1587 Florimond de Raemond, a magistrate in the parlement de Bordeaux and an antiquary published his first deconstruction of the legend, Erreur populaire de Pape Jane, which he followed with expanded editions the following year and in 1594. The tract applied humanist techniques of textual criticism to the Pope Joan legend, with the broader intent of supplying sound historical principles to ecclesiastical history, and the legend began to come apart, detail by detail. Raemond's Erreur populaire went through fifteen editions, as late as 1691.)
Since the 14th century, the figure of Pope Joan has taken on a somewhat "Saintly" figure. There are stories of her figure appearing and performing miracles. Francesco Petrarch (1304-74) wrote in his Chronica de le Vite de Pontefici et Imperadori Romani that after Pope Joan had been revealed as a woman:
"...in Brescia it rained blood for three days and nights. In France there appeared marvelous locusts which had six wings and very powerful teeth. They flew miraculously through the air, and all drowned in the British Sea. The golden bodies were rejected by the waves of the sea and corrupted the air, so that a great many people died." (Francesco Petrarch Chronica de le Vite de Pontefici et Imperadori Romani).
In 1675 a book appeared in English entitled A Present for a Papist: or the Life and Death of Pope Joan, Plainly Proving Out of the Printed Copies, and Manscriptes of Popish Writers and Others, that a Woman called JOAN, was really POPE of ROME, and was there Deliver'd of a Bastard Son in the open Street as She went in Solemn Procession. The book describes among other stories, an account of the purported Pope Joan giving birth to a son in plain view of all those around, accompanied by a detailed engraving showing a rather surprised looking baby peeking out from under the pope's robes. The book was penned "By a LOVER of TRUTH, Denying Human Infallibility." According to the preface the author had been "many years since deceased" and was "highly preferred in the Church of Rome." Furthermore, the preface indicates that the book was first printed in 1602.
The legend was also propagated in Chris Kuzneski's book Sign of the Cross which repeats the legend that Pope Joan died giving birth on the Roman streets.

Deconstructing the legend
Most scholars dismiss Pope Joan as a medieval legend.
This is all in agreement with the earliest known version of the legend, by Jean de Mailly, as he places the story in the year 1099. De Mailly's story was also acknowledged by his companion Stephen of Bourbon.
It has been argued that manuscripts and historical records were tampered with in the seventeenth century, when Pope Clement VIII decreed that there had never been a Pope Joan. But this claim is highly unlikely. It would have required an immense effort to remove her name from all documents, in every library and monastery across Europe. Such a vast conspiracy would have been almost impossible to carry out. Protestants would have assuredly protected evidence in their possession that disparaged the papacy. Moreover, any such tampering would be easily detectable by modern scholars. Either passages would have to be physically erased from manuscripts - something that obviously leaves marks - or the manuscripts would have to be completely destroyed and replaced with forgeries. However, scholars can date manuscripts quite accurately on the basis of the materials used, handwriting styles, and so on. There was no mass destruction, forgery or alteration of manuscripts in the seventeenth century.
Against the weight of historical evidence to the contrary, then, why has the Pope Joan story been so often believed, and so often revisited? Some, such as writer Philip Jenkins (The New Anti-Catholicism, 2005, ISBN 0-19-515480-0), have suggested that the periodic revival of what Jenkins calls this "anti-papal legend" has more to do with feminist and anti-Catholic wishful thinking than historical accuracy (pg. 89).
This article incorporates text from the public-domain Catholic Encyclopedia of 1913.

Analysis
The sedes stercoraria, the thrones with holes in it at St John Lateran's did indeed exist, and were used in the elevation of Pope Pascal II in 1099 (Boureau 1988). In fact, one is still in the Vatican Museums another at the Musée du Louvre. They do indeed have a hole in the seat. The reason for the hole is disputed, but as both the seats and their holes predated the Pope Joan story, and indeed Catholicism by centuries, they clearly have nothing to do with a need to check the sex of a pope. It has been speculated that they originally were Roman bidets or imperial birthing stools, which because of their age and imperial links were used in ceremonies by popes intent on highlighting their own imperial claims (as they did also with their Latin title, Pontifex Maximus).
Alain Boureau (Boureau 1988:23) quotes the humanist Jacopo d'Angelo de Scarparia who visited Rome in 1406 for the enthronement of Gregory XII in which the pope sat briefly on two "pierced chairs" at the Lateran: "the vulgar tell the insane fable that he is touched to verify that he is indeed a man" a sign that this corollary of the Pope Joan legend was still current in the Roman street.
Medieval Popes, from the thirteenth century onwards, did indeed avoid the direct route between the Lateran and St Peter's, as Martin of Opava claimed. However, there is no evidence that this practice dated back any earlier, let alone that it originated in the ninth century as a deliberate rebuff to the memory of the female Pope. The origin of the practice is uncertain, but it is quite likely that it was maintained because of widespread belief in the Joan legend and that it was thought genuinely to date back to that period.
Although some medieval writers referred to the female Pope as "John VIII", the real Pope John VIII reigned between 872 and 882, and his life does not resemble that of the fictional female Pope in any way.
A problem sometimes connected to the Pope Joan legend is the fact that there is no Pope John XX in any official list. It is sometimes said that this reflects a renumbering of the Popes to exclude the woman from history. Yet as historians have known since Louis Duchesne's critical edition of the Liber Pontificalis, this renumbering was actually due to a misunderstanding in the textual transmission of the official papal lists, where in the course of the 11th century, in the time after John XIX, the entry on John XIV had been misread as being referring to two different popes of this name, who then came to be distinguished as Iohannes XIV. and Iohannes XIV. bis ("John XIV the second"). The existence of a "second" pope John XIV was widely accepted in the 13th century, and by consequence the numbering of popes John XV through XIX was regarded as being erroneous. When Petrus Hispanus was elected pope in 1276 and decided for the papal name John, he meant to correct this error in enumeration by skipping the number XX and having himself counted as John XXI, thus acknowledging the presumed existence of John XIV "bis" in the 10th century who had nothing to do with the alleged existence of a pope John (Joan) VIII in the 9th century.

In popular culture

Legends surrounding the Papacy
Marozia Bibliography