others, 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 stutterers. 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 confirms 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 reduced when auditory feedback is altered, such as by 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. Brain scans of adult stutterers have found greater activation of the right hemisphere, which is associated with emotions, than of 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 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 exceed their capacity to deal with these pressures. However, the precise nature of the capacity or incapacity has not been delineated. Mechanism Physiology Though neuroimaging studies have not yet found specific neural correlates, there is much evidence that the brains of adults who stutter differ from the brains of adults who do not stutter. Several neuroimaging studies have emerged to identify areas associated