Yesterday I had a conference call with John Harrison the founder of Hexagon.
Stammering had a big impact on the first half of John Harrison's life. Like the ancient Greek philosopher whose whole life was spent trying to unravel a mystery, John's "compulsive nature", as he puts it, drew him into a deep understanding of his own stammering. The process helped him develop a broad view of the nature and treatment of stammering summed up in a model he has developed, called the 'stammering hexagon'.
Stammering Hexagon is shown as below:
My understanding from what he said was that these 6 factors are major principles that cause or cure stammering.
You can find the collection of his research and research of others which have been gathered for years here.