Modern research has indicated that one of the major factor or cause of stammering is that signal from brain (telling us to speak) is sometimers incorrectly transmitted to our motor system (the physical system by which we speak). This neuro-motor de-synchorisation results in stammering.
This may be due to genetic reason or otherwise. What happens is that when a baby starts growing up, the neuro-motor interaction does not develop smoothly. This explains why stammering is higher in children. slowly this neuro-motor system stabilises and in many cases becomes perfectly synchronised. but in few cases, this does not synchorise perfectly. this explains why only 1% adult stammers compared to 10% children.

Yes, sometimes few people do develop stammering in their adulthood. This might be due to a neuro-motor de-synchorisation occuring in adulthood. Why it occurs is still unknown.

Now let's talk briefly about the stammering iceberg. I wont talk much about it, as you will find more information about it online. Iceberg is characterised by the fact that 10% of the iceberg floats on the water and hence visible to us. For us that seems like the whole of iceberg. but actually 90% of the iceberg is under the water, which we dont see. Similarly the blocking and repetition in speech of a PWS is noticed by others, and for them this is what stammering is. but we all PWS know, its not as simple as that.

On top of out 'actual' stammering, which is the neuro-motor de-synchorisation, over the years we develop shame, fear, worry and other emotions which result in a psychological layer. This Psychological layer result in more stammering. This Psychological layer becomes very large during our teens and under pressure, that sometimes our actual neuro-motor de-synchorisation becomes insignificant but our Psychological layer is so big that we have 'significant' overall stammering (simple example is note how much fluently can you talk in front of a mirror and then in front of a crowd). On top of this, we develop ways of trying not to stammer- like the way we contour our face to force out the word, change breathing pattern to force the words out, tense our muscles etc , all in order to not to stammer, but actually results in more stammering. This is what we call 'Physiological' stammering. So the over-all stammeering becomes huge. Hence what is actually stammering (the neuro-motor de-synchorisation) is only a small part (tip of iceberg i.e. 10%) compared to the overall iceberg.

It is very difficult to 'cure' this neuro-motor de-synchorisation. However one must note that we can always work on this psychological and physiological stammering.
Proper breathing techniques and other things speech therapist use, work on the physiological stammering. I would put yoga and other therapies in this category too.
NLP, probably yoga, self acceptance, building up of confidence etc work on the psychological stammering. Then we are left with the small part-the actual stamemring due to neuro-motor de-synchorisation and we then notice that this neuro-motor de-synchorisation is actually very tiny and is most cases, we will be happy to live with this. I know its not easy to remove the psychological and physiological stammering, as it has become a habit, a sub-concious part of us, but its not impossible. Many recovering PWS has proven that.

There has been some misconceptions I would like to clear up:
Stammering does not arise from imitating a stammerer, even by a child. This misconception has led to prevention of stammering children to be befriended by other non-stammering children vby parents of the non-stamemering children. There is no scientific evidence that imitating another stammering can result in stammering.

Stammering does not have to genetic. Nobody in the family might have stammering but you might have it. That does not rule out genetics though. It is possible that there might be someone generations above you (whom no one knows about today) and the gene is passed to you (and many others) and somehow in you the gene has become active or so to say dominant.

Nevousness is not the original cause of stammering. Yes, a PWS feels nervous because of his stammering and this ofcources increases his stammering, but is not the original cause.

Stammering does not originate from any fear. Yes, some severe trauma or shock may result in neuro-motor de-synchorisation, but usually the person would recover. This is why some people who suffer from stroke develop stammering. Many people belive that the develop stammer after a fearful incident in childhood, but this is usually not true.

-Tanveer Rameez Ali, PWS and ex-Trustee of British Stammering Association (2007-2010)