The incidence of pediatric fluency disorder refers to the number of new cases identified in a specific time period. The prevalence refers to the number of individuals who are living with fluency disorders in a given time period. Estimates of incidence and prevalence vary due to a number of factors, including disparities in the sample populations (e.g., age), how stuttering was defined, and how it was identified (e.g., parent report, direct observation).
Approximately 5% of people will stutter during some part of their lives (Mansson, 2000). However, in a review of recent research, Yairi and Ambrose (2013) concluded that the lifetime incidence of stuttering may be higher. They suggested that past underdiagnosis may be due to self-reporting and poor subject selection. An increased incidence of stuttering has been noted among those with a first degree relative (e.g., parent, sibling) who stutters and an even greater likelihood if that relative is an identical twin (Kraft & Yairi, 2011).
Higher incidence rates have been noted with preschool-age children with findings suggesting that incidence reaches a cumulative rate of 8.5% by 3 years of age (Reilly et al., 2009) and 11% by 4 years of age (Reilly et al., 2013). The incidence of stuttering in young boys is approximately twice that of girls (Yairi & Ambrose, 1999), although some studies report a smaller ratio of 1.6:1.0 (Kloth, Janssen, Kraaimaat, & Brutten, 1995; Mansson, 2000). As stuttering becomes more persistent, this difference becomes more pronounced, and by school age the number of boys who stutter becomes 3 to 4 times more than the number of girls (Craig, Hancock, Tran, Craig, & Peters, 2002).
The prevalence rate for stuttering across all age groups has been estimated to be 0.72% (Craig et al., 2002). Most reports indicate a higher prevalence in children, with estimates ranging from 0.9% to 5.6% (Craig et al., 2002; McLeod & Harrison, 2009), depending on the age of the children included. Data collected through the Centers for Disease Control and Prevention (CDC) indicate a prevalence rate of 1.6% for children between 3 and 17 years of age (Boyle et al., 2011). Significantly higher prevalence rates were reported for children ages 3-10 than for those ages 11-17 (i.e., 1.99 versus 1.15).
Researchers have reported conflicting findings for the effects of race or ethnicity on the prevalence of stuttering in children; some studies show a disparity based on race, but others do not (Boyle et al., 2011; Proctor, Yairi, Duff, & Zhang, 2008).
Higher prevalence rates for fluency disorders have been reported in children with genetic syndromes, such as Down syndrome and fragile X (Van Borsel & Tetnowski, 2007).
Experts in the field of cluttering have consistently estimated that approximately one third of children and adults who stutter also present with at least some components of cluttering (Daly, 1986; Preus, 1981; Ward, 2006). It is important to note that there are more clinical anecdotes than data to support this statement; further research on the incidence and prevalence of cluttering is needed (Scaler Scott, 2013).