Why is it that kids set up secret societies and then make sure that certain of their 'friends' don't know the password to get in? Adults do the same thing, but they call it "earning your privileges" or "loyalty rewards".

Human beings seem to like to feel that they are on the 'inside' while others remain in the dark. It is not a pleasant character trait.

The insiders in The Da Vinci Code belong to the Priory of Sion. The Priory is very much the key to the unfolding mystery of the novel. It is the secret society that holds the knowledge of 'true Christianity' that has been hidden from the Church and its adherents for 2000 years. In The Da Vinci Code a list of supposed Grand Masters of the Priory of Sion is presented, among them Leonardo da Vinci and Isaac Newton, not to mention the dead art curator and riddle-weaver, Jacques Sauniere.

At this point, many readers have responded with something like: "Yes! The Priory sounds plausible to me. I can easily believe in a secret society from the Middle Ages that knew things about Jesus that have since been suppressed. It feels believable. What's more, there are all sorts of documents recording its history-aren't there?"

This sense that something like the existence of the Priory could be the case-let's call it the conspiracy vibe-fits rather well with our culture's expectation that there is someone, somewhere, behind the scenes pulling the strings. But in this case, it is based on very little other than the vibe itself. Our sense that 'it feels right' is an 'Aringarosa'-a 'red herring'.

In one sense, the Priory has a basis in fact; at least, the name has a few traceable origins. There are shadowy scraps of information about a monastic order with a similar name, founded in Jerusalem in 1100 and absorbed into the Jesuits in 1617, but nothing solid. The hard evidence for the Priory of Sion comes into view because of a figure called Pierre Plantard, a French anti-Semite who wished to purify France and believed he had a rightful claim to the French throne.

In 1956, Plantard formed a group called the Priory of Sion with a handful of friends with a similar mindset. This group set about fabricating a set of documents that would prove the existence of a bloodline descending through the medieval kings of France to Plantard himself. Having forged this alternative history, Plantard and his colleagues planted the documents in libraries all over France, including the National Library. Some British journalists, Henry Lincoln, Richard Leigh and Michael Baigent, dug up the material and added to it the story that it was in fact the blood of Mary Magdalene that ran through the veins of the kings of France, thus pulling together myth on myth. They wrote up the whole imaginary saga in their bestselling book of the early 1980s, Holy Blood, Holy Grail. It is this book that provides Dan Brown with much of his material for the stories behind The Da Vinci Code.

In 1993, Plantard admitted under oath to a French judge that he had fabricated all the documents relating to the Priory of Sion. The judge warned him not to toy with the judicial system and dismissed him. He died in 2000.

No actual Priory of Sion dating back to the Middle Ages. No list of Grand Masters who knew the secrets about Jesus and Mary Magdalene. No special clues to this spiritual secret in the artwork of Leonardo Da Vinci.

There isn't much of a secret society left to join.