Causes of stuttering are multifactorial and include certain genetic and neurophysiological factors that are thought to contribute to its emergence. Although acquired neurogenic and psychogenic stuttering are not covered here, they may exist in rare cases in children. It is presumed that each child who stutters develops stuttering as a result of his or her own unique factors. Theories regarding the onset of stuttering are diverse.
Despite popular beliefs, emotional problems and parenting style do not cause stuttering. However, coping with stuttering can result in significant emotional reactions and avoidance behavior. Specifically, emotional reactivity/regulation and behavioral disinhibition have been found to be associated with stuttering and may affect the child's ability to cope with disfluencies (Choi, Conture, Walden, Lambert, & Tumanova, 2013; Jones, Conture, & Walden, 2014; Ntourou, Conture, & Walden, 2013). Furthermore, although not considered a cause of stuttering, environmental factors may exacerbate disfluency. These factors can include family dynamics, fast-paced lifestyle, stress and anxiety, and the child's temperament (Anderson, Pellowski, Conture, & Kelly, 2003). Alm (2014) indicates that "children who develop stuttering (as a group) are not characterized by temperamental traits such as shyness, social anxiety, or general anxiety" (p. 18). For a review of temperament, emotion, and childhood stuttering, see Jones, Choi, Conture, and Walden, 2014.
While genetics and neurophysiology appear to be related to the underlying causes of stuttering, environmental factors, temperament, and speaking demands may influence a child's reactions to stuttering.
In a review of the literature on the genetics of stuttering, Kraft and Yairi (2011) indicated that there is support from a range of studies for a genetic perspective of stuttering, but that "no definitive findings have been made regarding which transmission model, chromosomes, genes, or sex factors are involved in the expression of stuttering in the population at large" (p. 34).
Studies have identified likely causative gene mutations linked to stuttering. Mutations to three genes (GNPTAB, GNPTG, and NAGPA) have been found to disrupt the signal that directs enzymes to their target location in the lysosome of the cell (Drayna & Kang, 2011). Drayna and Kang (2011) found that gene mutations were present in close to 10% of cases of familial stuttering.
Recent studies have shown both structural and functional neurological differences in children who stutter (Chang, 2014). Neurophysiological factors that are thought to contribute to stuttering include the following.
- Gray and White Matter Differences
Children with persistent stuttering showed deficiencies in left gray matter volume with reduced white matter integrity in the left hemisphere. In contrast to adults who stutter, children who stutter did not show increases in white matter tracts in the right hemisphere (Chang, Erickson, Ambrose, Hasegawa-Johnson, & Ludlow, 2008).
- Neural Network Connectivity Differences
Children who stutter (ages 3 to 9 years) have reduced connectivity in areas that support the timing of movement control. These differences may affect speech planning needed for fluency (Chang & Zhu, 2013).
- Atypical Lateralization of Hemispheric Functions
Preschool children who stutter showed differences in event-related brain potentials used as indices of language processing. These findings suggest the presence of atypical lateralization of speech and language functions near the onset of stuttering. These brain differences have previously been observed in adults who stutter (Weber-Fox, Wray, & Arnold, 2013).
- White Matter Connections
Adolescents and young adults who stutter were found to have more white matter connections in the right hemisphere as compared with normally fluent controls (Watkins, Smith, Davis, & Howell, 2008).
The above neurophysiological factors should be interpreted with caution due to the small number of subjects and the heterogeneity of the methodologies used. Furthermore, the different areas of the brain studied or the technologies used to conduct brain research also varied widely (e.g., PET, MEG, MRI, fMRI, NIRS, DCS). Another caution is to consider differences between children and adults when interpreting data from neurological studies. Chang et al. (2008) highlight "the importance of considering the role of neuroplasticity during development when studying persistent forms of developmental disorders in adults" (p. 1333).
Risk Factors For Persistent Stuttering
It is not possible to determine with certainty which children will continue to stutter, but there are some factors that indicate a greater likelihood of chronicity. Risk factors that may be associated with persistent stuttering include
- sex of child, as boys are at higher risk for persistence of stuttering than girls (Craig et al., 2002; Yairi & Ambrose, 2013);
- family history of persistent stuttering (Kraft & Yairi, 2011);
- time duration since onset of greater than 6 to 12 months or no improvement over several months (Yairi & Ambrose, 2005);
- age of onset-children who start stuttering at age 3½ or later (Yairi & Ambrose, 2005);
- co-occurring speech and language impairment (Ntourou, et al., 2011; Yaruss et al., 1998).
With regard to cluttering, research is not far enough along to identify causes. There is very little genetic information on cluttering, except for anecdotal reports that the speech characteristics have been found to be present in more than one member of a family (Drayna, 2011).
Features of cluttering are sometimes observed in conjunction with other neurological disorders (e.g., ASD, Tourette's, and ADHD). Potential neurological underpinnings of cluttering include dysregulation of the anterior cingulate cortex and the supplementary motor area (Alm, 2011).
Speech Production/Self-Regulation Factors
Systems that govern self-regulation may underlie cluttering; qualitative interviews with those who clutter suggest that thoughts emerge before they are ready (Scaler Scott & St. Louis 2011). The speaker is proposed to be talking at a rate that is too fast for his or her system to handle, resulting in breakdowns in fluency and/or intelligibility (Bakker, Myers, Raphael, & St. Louis, 2011).
Risk Factors For Cluttering
There is not enough epidemiological research to state specific risk factors for cluttering. Unlike stuttering, there are no data regarding age since onset and long-term outcome of cluttering. Additionally, there is no documented recovery from cluttering; therefore, duration since onset does not seem to apply as a risk factor. Potential risk factors for cluttering include
- sex of child—it appears that the disorder in more common in males than in females; the male-to-female ratio for cluttering has been reported to range from 3:1 to 6:1 (Arnold, 1960; St. Louis & Hinzman, 1986; St. Louis & Rustin, 1996);
- family history—anecdotal reports of cluttering being present in more than one member of a family indicate that having others in the family with cluttering may be a risk factor;
- presence of co-occurring disorders
- given that cluttering may co-occur with other disorders-such as learning disabilities, ASD, and Tourette's syndrome, having any of these disorders may be a risk factor; however, not all individuals with these disorders also exhibit cluttering;
- presence of stuttering-an estimated one third of people who stutter also present with at least some components of cluttering (Daly, 1986; Preus, 1981; Ward, 2006).
Speaking two languages at home since birth does not cause stuttering (Shenker, n.d.). Information is varied and conflicting regarding the exact relationship between bilingualism and disfluencies (Van Borsel, Maes, & Foulon, 2001; Tellis & Tellis, 2003). Bilingualism may influence stuttering in unpredictable ways due to variations across social settings in language selection and fluency (Foote, 2013). Clinicians must consider other psychosocial issues that may lead to the onset of stuttering or a temporary increase in stuttering in multilingual individuals, including the effects of being in new and unfamiliar situations, using a new language, and being exposed to mixed linguistic input (Shenker, 2013).