The exact cause of stammering is still unclear. Most experts think there are several possible factors that can trigger stammering in children.
These factors are described below.
Cases of stammering have been known to run in families. For example, if an identical twin has a stammer, there is a high chance that the other twin will also stammer.
This suggests that the genes a child inherits from their parents might cause them to stammer.
Research carried out in 2010 identified a number of specific genetic mutations (faulty genes) common in adults of Pakistani origin with a persistent stammer who had a family history of stammering.
It is unlikely there is one gene responsible for stammering. It is more likely that a problem in acquiring speech and language runs in families, which manifests itself in stammering behaviour.
There is evidence that stammering is four times more common in males than females. This links with other research about developmental disorders, which boys appear to be more likely to experience than girls. Girls are also more likely to see their stammers resolve naturally than boys.
Cognitive activity is a medical term used to describe many different mental processes that occur in the human brain.
Researchers have used magnetic resonance imaging (MRI) scans to study the brains of adults with a persistent stammer. MRI scans use strong magnetic fields and radio waves to produce a detailed image of the inside of the body.
The brain scans revealed that the pattern of cognitive behaviour was different in the brains of adults who stammer. In particular, there were:
- low levels of activity in a part of the brain called the left temporal lobe, which is involved in processing sound and speech
- unusually high levels of activity in the right hemisphere (in most adults, the left hemisphere is active when a person is speaking)
These results have led to a number of theories about how these abnormal patterns of cognitive activity may contribute to stammering.
First, the low levels of activity in the left temporal lobe may mean that a person has an impaired feedback system. This may cause differences in the way they hear their own voice.
Second, the abnormal activity in the right hemisphere may mean that a person who stammers has interference from this part of the brain when they are speaking.
However, it is unclear whether these effects are the result of stammering or the cause of stammering.
Another area of research has focused on a part of the brain known as the basal ganglia. The basal ganglia is responsible for some of the most basic human abilities and for processing powerful emotions such as fear and aggression.
Brain scans of adults who stammer have shown unusual activity in the basal ganglia. It may be that their stammering is caused by a problem with regulating the functions of the basal ganglia.
It is not known if this unusual cognitive activity is found in children who stammer, as no brain scans involving children have been carried out.