There are different types of stuttering. Neurogenic stuttering happens as a result of brain injury like stroke, and psychogenic stuttering is associated with psychiatric disease. But the most common type is developmental stuttering, which appears in children 2-4 years old as they are learning to speak. This type of stuttering is mainly genetic; some figures claim that it is up to 70% heritable. Scientists can determine heritability by performing twin studies, looking at the difference in frequency of the disorder between fraternal and identical twins.
Stuttering can become worse in stressful situations, but at its core, it is generally considered to be neural and not just due to anxiety. Developmental stuttering is associated with many brain structural abnormalities. In particular, the left inferior frontal gyrus of children who stutter has abnormal gray matter, and less white matter connecting it to the rest of the brain. This region is normally involved in language production, and is loosely related to the speech production region popularly known as “Broca’s area”. To learn more, visit the Stuttering Foundation.