'What causes stammering?' Most experts agree that it is due to a combination of factors - physiological, neurological, psychological and environmental - but the precise ways that these factors operate together is not known, and clearly the combination will be different from one individual to another.
What is clear, then, is that stammering is a complex condition with many characteristics affecting each individual in a unique way. Whilst two people who stammer may sound similar to an outsider, the factors that trigger stammering, and reactions to stammering (both physical and emotional) can be very different. These aspects are highly personal and make it important to consider individual needs in any course of therapy. (The British Stammering Association)