By Christine Weber-Fox, Ph.D., and Anne Smith, Ph.D., Purdue University
We consider stuttering as a neurodevelopmental, epigenetic, multifactorial disorder.  In our laboratory, we take a multifactorial approach to studying speech motor, language, and emotional measures, both physiological and behavioral/clinical, to understand which factors in these broad domains are important in the development of stuttering, and which will help predict recovery or persistence of stuttering. 
It is clear that each child will have a unique profile on these factors and that the importance of various factors will differ across children. For example, some children who stutter show delayed language development, while others do not. 
We know that stuttering develops over time because monozygotic (identical) twins are only about 50% concordant for stuttering. This means that in two human beings with identical genes, one may become a person who stutters, while his/her twin does not. Epigenetics is an emerging field of science that bridges the gap between genes and environment. Epigenetic scientists study the control processes that determine the timing and degree of activation and deactivation of genes. This is a critical element in whether the person will ultimately have the trait or disorder of interest.  Applied to stuttering this means that two individuals may inherit the identical genetic propensity to stutter, but the process of epigenises ultimately determines if the individual in fact develops a persistent stuttering problem.  For children who go through a period of stuttering, this epigenetic process involves recruiting new areas of the brain (for example right brain areas)  to compensate for the left brain anomalies we know are associated with school- age and adult stuttering.  Much evidence points to the conclusion that if this brain compensation is adequate, the child recovers from stuttering.
The neurodevelopmental aspects of stuttering are apparent when we consider that the disorder does not exist at birth, rather it unfolds with the development of the brain. Stuttering onset occurs during the preschool years (average age of onset 33 months) a period when neural circuits supporting language and speech production undergo tremendous growth and interaction. At this time, the number of neural connections in the brain is overabundant, far outnumbering that of adolescents and adults.  Each individual child’s experiences and behaviors will ultimately determine how his/her neural connections and brain functions are ‘fine-tuned’. Around the time of stuttering onset, the child’s language abilities are growing rapidly, putting additional linguistic and speech motor planning demands on the child.  Also, around this time, the child is just beginning to acquire the ability to inhibit maladaptive behaviors suggesting a dramatic change in psychosocial skills.  A significant aspect of understanding stuttering as a process unfolding over development is that we emphasize that stuttering “does not live” in one small part of the brain, rather stuttering arises not from one faulty circuit, but from the atypical interaction of many circuits, so that the adult-like, stable motor patterns of speech production achieved by typically developing children during the teenage years are not achieved in children with a persistent stuttering problem. 
Major advances have been made in understanding the behavioral and physiological characteristics of stuttering close to its onset. We direct the Purdue Stuttering Project,, and are first recruiting children when they are 4-5 years old. We follow each child for 5 years to determine which of the children ultimately recover or persist. In addition to careful assessment of stuttering behaviors, standardized speech and language testing, our project includes physiological protocols, including recording of speech motor activity and brain wave indices of the neural functions for language processing. Our findings to date from preschool children demonstrate that  speech motor systems of the stuttering group are less mature compared to their peers and that they are already showing signs of atypical hemispheric activation for language tasks. In addition we now have evidence that some clinical language measures as well as indices of speech motor coordination may help predict which children who are stuttering at age 4-5 will recover and who will persist.   Our findings also suggest that neural functions for processing the sounds of our language, may still be atypical in children who have recovered from stuttering by the age of 7-8 years, that is if there is still a neural  ‘imprint’ of a history of stuttering in these children. In contrast the children who persist at that age show even greater atypical neural patterns. 
The Theory. A central tenet of the theory has been outlined above: stuttering develops as the growth of neural networks to support language and speech processes form atypical brain regions and connections. The majority of preschool children recover from stuttering. We hypothesize that those who develop chronic stuttering do so as a result of the complex interactions among a number of factors, including motor, language, and emotional factors. Through epigenesis, the growth of the brain is influenced by environmental factors that play a critical role in determining the child's path to persistence or recovery. An important part of our effort is focused on translating our experimental work in the motor, language, and emotional domains to develop a test battery that speech/language pathologists can use to assess the risk for persistent stuttering in preschool children. We believe that those children who are likely to persist should be provided treatment as soon as possible, when their brain functions are the most malleable. 
From the 2014 Summer Newsletter