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There is no known cause for stuttering. Theories about the causes of stuttering can be divided into three categories.
Stuttering could possibly be inherited, and 50% to 70% of all stutterers are related to another stutterer. 8 9 While having a stutterer in the family does not automatically create another stutterer, it has been shown to create "stuttering potential" or a "stuttering predisposition." This inherited genetic factor may cause the speech pathways in the brain to be less efficient, making it difficult for the stutterer to meet fluency demands and get his or her words out quickly. Genetics may also influence the temperament of some stutterers, which makes them react negatively to their own early stuttering behavior. In a 1999 study conducted jointly between U.S. and Australian researchers, hundreds of twins who stuttered were examined, with significant differences in concordance rates for stuttering being found between identical and fraternal twin pairs. Scientists are also working to identify the "stuttering genes", largely through the Stuttering Research Project at the University of Illinois. |
In the past, the cause of stuttering was most
often attributed to events during childhood development. Some
argued that neurotic conflict or disturbed interpersonal relationships
during crucial stages of a child's development could create a
stutter. Others said that stuttering develops from the normal
mistakes all children make when learning to speak, when some children
get caught in a vicious cycle of putting too much effort into
speaking, tensing their speech-production muscles too much, and
consequently worsening their speech. While such activity does
worsen an already present stutter, it does not create one. Today,
these theories are generally disregarded in favor of the genetic
and neurophysiological models.
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In 1939, a controversial study on the possibility of "creating a stutterer" was conducted by University of Iowa speech pathologist Wendell Johnson and his graduate student Mary Tudor. The study tried to create stutterers over the course of 4 months, using 22 unwitting orphans from the Soldiers and Sailors Orphans’ Home in Davenport, Iowa. Ethically acceptable at the time, it was designed to induce stuttering in normally fluent children and to test out Johnson's "diagnosogenic theory"—a theory suggesting that negative reactions to normal speech disfluencies cause stuttering in children. The study divided the orphans into 3 groups: 6 normally fluent orphans would be given negative evaluations and criticisms regarding their speech, another group of 5 orphans who allegedly already stuttered would also receive that treatment, and the remaining 11 would be treated neutrally. The study concluded that the children given negative evaluative labeling went on to develop persistent, permanent stutters. The study was influential at the time, with many speech pathologists and child-health and educational professionals accepting Johnson's theory. In 1988, Silverman first reported the results of this study in the Journal of Fluency Disorders and labeled it "The Monster Study." While the information was well known to professionals in the area of stuttering, it was not until 2001 when the information was distributed to the public in a newspaper article. In June 2001, the San Jose Mercury News revealed this study to the public for the first time, leading to widespread controversy and debate about scientific ethics. Soon after, University of Illinois professors Nicoline Ambrose and Ehud Yairi wrote a paper discrediting the 1939 study, revealing flaws in data collection and method, as well as pointing out that none of the orphans actually did develop a permanent stutter. While criticism of a developing child's speech can certainly make a present stutter worse, it does not create a stutter.
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Another prominent view is that stuttering is caused by neural synchronization problems in the brain. Recent research indicates that stuttering may be correlated with disrupted fibers between the speech area and language planning area, both in the left hemisphere of the brain. Such a disruption could potentially be due to early brain damage or to a genetic defect.
Structural neuroimaging, where the anatomy of the brain can be visualized and analyzed, has shown an anatomical difference between the brains of those who stutter and those who do not: see research by Sommer et al. and Jaehnke et al. And, functional neuroimaging, where processes in the brain can be observed, has shown some differences in the state of stuttering. Differences in brain activity have been observed in other areas that are associated with speech motor function, such as the area of the primary motor cortex that controls mouth movements, the areas associated with perceiving and decoding sounds, the areas involved with the formulation and expression of language, and the area that controls the human body's Valsalva mechanism
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