Stuttering - Causes of Developmental Stuttering

Causes of Developmental Stuttering

No single, exclusive cause of developmental stuttering is known. A variety of hypotheses and theories suggests multiple factors contributing to stuttering. Among these is the strong evidence that stuttering has a genetic basis. Children who have first-degree relatives who stutter are three times as likely to develop a stutter. However, twin and adoption studies suggest that genetic factors interact with environmental factors for stuttering to occur, and many people who stutter have no family history of the disorder. There is evidence that stuttering is more common in children who also have concomitant speech, language, learning or motor difficulties. Robert West, a pioneer of genetic studies in stuttering, has suggested that the presence of stuttering is connected to the fact that articulated speech is the last major acquisition in human evolution. Another view is that a stutter is a complex tic.

In a 2010 article, three genes were found to correlate with stuttering: GNPTAB, GNPTG, and NAGPA. Researchers estimated that alterations in these three genes were present in 9% of people who stutter who have a family history of stuttering.

For some people who stutter, congenital factors may play a role. These may include physical trauma at or around birth, as well as cerebral palsy and mental retardation. For other people who stutter, there could be added impact due to stressful situations such as the birth of a sibling, moving, or a sudden growth in linguistic ability.

There is clear empirical evidence for structural and functional differences in the brains of people who stutter. Research is complicated somewhat by the possibility that such differences could be the consequences of stuttering rather than a cause, but recent research on older children confirm structural differences thereby giving strength to the argument that at least some of the differences are not a consequence of stuttering.

Auditory processing deficits have also been proposed as a cause of stuttering. Stuttering is less prevalent in deaf and hard-of-hearing individuals, and stuttering may be improved when auditory feedback is altered, such as masking, delayed auditory feedback (DAF), or frequency altered feedback. There is some evidence that the functional organization of the auditory cortex may be different in people who stutter.

There is evidence of differences in linguistic processing between people who stutter and people who do not stutter. Brain scans of adult people who stutter have found increased activation of the right hemisphere, which is associated with emotions, than in the left hemisphere, which is associated with speech. In addition reduced activation in the left auditory cortex has been observed.

The capacities and demands model has been proposed to account for the heterogeneity of the disorder. In this approach, speech performance varies depending on the capacity that the individual has for producing fluent speech, and the demands placed upon the person by the speaking situation. Capacity for fluent speech, which may be affected by a predisposition to the disorder, auditory processing or motor speech deficits, and cognitive or affective issues. Demands may be increased by internal factors such as lack of confidence or self esteem or inadequate language skills or external factors such as peer pressure, time pressure, stressful speaking situations, insistence on perfect speech, and the like. In stuttering, the severity of the disorder is seen as likely to increase when demands placed on the person's speech and language system is exceeded by their capacity to deal with these pressures.