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Purpose The purpose of this study was to assess the relation of behavioral inhibition to stuttering and speech/language output in preschool-age children who do (CWS) and do not stutter (CWNS). Method Participants were preschool-age (ages 36–68 months), including 26 CWS (22 males) and 28 CWNS (13 males). Participants’ behavioral inhibition (BI) was assessed by measuring the latency to their sixth spontaneous comment during conversation with an unfamiliar experimenter, using methodology developed by Kagan, Reznick, and Gibbons (1989). In addition to these measures of BI, each participant's stuttered and non-stuttered disfluencies and mean length of utterance (in morphemes) were assessed. Results Among the more salient findings, it was found that (1) there was no significant difference in BI between preschool-age CWS and CWNS as a group, (2) when extremely high versus low inhibited children were selected, there were more CWS with higher BI and fewer CWS with lower BI when compared to their CWNS peers, and (3) more behaviorally inhibited CWS, when compared to less behaviorally inhibited CWS, exhibited more stuttering. Conclusions Findings are taken to suggest that one aspect of temperament (i.e., behavioral inhibition) is exhibited by some preschool-age CWS and that these children stutter more than CWS with lower behavioral inhibition. The present results seem to support continued study of the association between young children's temperamental characteristics and stuttering, the diagnostic entity (i.e., CWS versus CWNS), as well as stuttering, the behavior (e.g., frequency of stuttered disfluencies). Educational objectives: After reading this article, the reader will be able to: (a) summarize the salient empirical findings in the extant literature with regard to the association between temperament and childhood stuttering; (b) describe the concept of behavioral inhibition (BI) as well as the methods to measure BI; and (c) discuss the association between behavioral inhibition and childhood stuttering in preschool-age children.

1.1. Temperament and developmental stuttering Recent reviews suggest that increased attention is being paid to the possible association between temperament and childhood stuttering (Conture et al., in press, Kefalianos et al., 2012 and Seery et al., 2007). Temperament, as defined by Rothbart and Bates (1998), can be described as “constitutionally-based individual differences in emotional, motor and attentional reactivity and self-regulation…(that are) relatively stable over time” (p. 109). Sanson, Hemphill, and Smart (2004) further suggest that such “…constitutionally-based differences in behavioral style…are visible from the child's earliest years” (p. 143). These constitutionally or biologically based components include genetic as well as non-genetic elements such as prenatal, environmentally based variables (e.g., prenatal drug exposure), birth complications, and perinatal influences present in the child's early rearing environment. Regarding the possible association between temperament and childhood stuttering, in a recent review, Kefalianos et al. (2012) cautiously concluded that there may be some association between temperament and stuttering during the preschool years. The cautious nature of their conclusion resulted from several factors, none the least of which being the relatively small number of published studies as well as inconsistencies among findings. However, using independent replication of findings as a guideline for “trustworthiness” of findings, Kefalianos et al. noted some consistencies, that is, preschool-age children who stutter (CWS), when compared to preschool-age children who do not stutter (CWNS), appear to exhibit (1) lower adaptability (Anderson et al., 2003, Howell et al., 2004 and Schwenk et al., 2007) (2) lower attention span/persistency (Howell et al., 2004, Karrass et al., 2006 and Schwenk et al., 2007), (3) more negative quality of mood (Howell et al., 2004 and Johnson et al., 2010) and (4) higher activity level (Embrechts et al., 2000 and Howell et al., 2004). In addition, several empirical studies, not reported in the above review, have also shown that CWS, when compared to CWNS, are more emotionally reactive to environmental stimuli (Karrass et al., 2006 and Wakaba, 1998) as well as lower in inhibitory control (i.e., the capacity to plan and suppress inappropriate approach responses under instructions or in novel or uncertain situations) and attention shifting (Eggers, De Nil, & Van den Bergh, 2010). In brief, various aspects of temperament appear to be associated with childhood stuttering, involving attention, affect/mood, adaptability, reactivity to their environment, and inhibitory control. Such between-group differences, although clearly warranting further empirical assessment, as suggested by Kefalianos et al. (2012), may reflect real differences between CWS and CWNS based on Eggers, De Nil, and Van den Bergh's (2009) findings. Specifically, Eggers et al. reported a similar, highly congruent three-factor temperament structure for children who stutter, children who do not stutter and children with vocal nodules. Eggers et al. concluded that any possible differences between CWS and CWNS on various indexes of temperament reflect real differences and are not the result of differences in underlying temperamental construct(s). Based on the findings reviewed above, it has been theorized that the temperamental processes of children who stutter may also contribute to the difficulties these children have establishing normally fluent speech (Conture and Walden, 2012, Conture et al., 2006 and Walden et al., 2012). For example, Conture et al.’s (2006) Communication–Emotion (C–E) model suggests that temperamental factors (e.g., emotional reactivity or emotion regulation) may exacerbate the speech disfluencies of children who stutter. According to the C–E model, emotional reactivity may be associated with detecting/reacting to speech errors whereas emotion regulation may be related to changing, correcting and/or coping with covert/overt speech/language errors. These responses, according to the C–E model, are thought to contribute to quantitative (e.g., frequency) and/or qualitative (e.g., types and duration) change in stuttering. More recently, Conture and Walden (2012) proposed a dual diatheses-stressor framework (DD-S), in which diatheses (i.e., vulnerabilities) and stressors relating to emotion and speech-language processes are associated with childhood stuttering (for general review of diathesis-stress models, see Monroe & Simons, 1991). The DD-S model predicts that emotional reactivity, emotional regulation and their joint effects impact the frequency and severity of stuttering in preschool-age children ( Walden et al., 2012). 1.2. Behavioral inhibition and developmental stuttering To date, most studies of temperamental characteristics associated with childhood stuttering have relied upon caregiver reports (e.g., Eggers et al., 2009 and Eggers et al., 2010). Although questionnaires measuring temperamental characteristics have been shown to be valid (Rothbart, 2011, Rothbart and Bates, 1998 and Thompson, 1999), it has been suggested that parents are biased informants (Strelau, 1998), and that a parent's report about a child's behaviors may not always yield the truest representation of the child's actual behaviors (Kiel & Buss, 2006). Thus, as Kagan (2007) suggests, in order to best understand the temperament of individuals, one should assess temperament from at least three perspectives: parental report, behavioral observation, and psychophysiology. Given that there is a relative lack of research relating temperament and childhood stuttering using methods other than parental reports, one reasonable next step would be to examine temperamental dimensions of children by means of behavioral observation. Kagan and his colleagues (Kagan et al., 1984, Kagan et al., 1989 and Kagan et al., 1998) reported that one temperamental characteristic, behavioral inhibition (BI), can be measured reliably by means of behavioral observation. Behavioral inhibition is a temperamental characteristic that is expressed as initial avoidance, distress, or subdued emotion when a person is exposed to unfamiliar people, places, and situations (Kagan et al., 1984 and Kagan et al., 1998). Behavioral inhibition has been considered to be a salient temperamental trait because of its similarity to an animal's immobility when encountering a novel context, a biologically prepared response thought to involve a state of fear (Blanchard & Blanchard, 1988). Kagan (1989) estimated that 10–15% of children are consistently inhibited and the same percentage of children are consistently uninhibited. To measure BI of 4 year-old children, Kagan et al. (1989) used the child's latency to the 6th spontaneous comment and the total number of spontaneous comments during a speaking task. Their findings indicated that for the entire sample, there was no preservation of inhibited or uninhibited behaviors from 14 or 20 months to 4 years of age. However, for the extreme sample (top and bottom 20 percentiles), there was significant preservation of the two categories of behaviors (i.e., inhibited and uninhibited behaviors) from the second to the fourth year of life. Accordingly, they speculated that the construct of being inhibited and uninhibited to the unfamiliar is a qualitative category rather than continuous dimension. Behavioral inhibition has also been found to be associated with children's reactivity (Calkins et al., 1996 and Kagan et al., 1998), which has been studied in association with childhood stuttering (e.g., Arnold et al., 2011, Johnson et al., 2010 and Walden et al., 2012). For example, using behavioral observations, Kagan et al. (1998) reported that 4-year-old children, who had been classified as low reactive at 4 months, talked more often than 4-year-old children classified as high reactive infants. Based on these findings, Kagan et al. argued that there is a predictive relation between reactivity and behavioral inhibition. According to Kagan et al., the basic assumption underlying their argument is that behavioral inhibition is a “derivative” of reactivity because both reactive infants and inhibited children are thought to possess a low threshold of activation in the central nucleus of the amygdala and its projections to the hypothalamus, sympathetic chain, and cardiovascular system. Although reactivity can be observed/measured in infants as young as 4 months of age, young infants do not display marked differences in shyness toward strangers or timidity to novel events until they turn 9-12 months ( Kagan and Snidman, 1991 and Kagan et al., 1998). Thus, Kagan and his colleagues’ speculation seems reasonable that a child's behavioral inhibition can be predicted by the child's reactivity as an infant rather than the other way around. It should also be noted that most studies reviewed above regarding the association between temperament and childhood stuttering have focused on between-group (i.e., CWS vs. CWNS) differences in temperamental characteristics. Given that the C–E model suggests that temperamental factors may exacerbate CWS’ speech disfluencies, a better understanding of the role of the temperamental characteristics in the development of stuttering might result from assessing how BI impacts the quantity of stuttered disfluencies within the group of CWS as well as CWNS. In the same vein, it would be interesting to investigate how BI impacts non-stuttered disfluencies (e.g., revisions) within CWS as well as CWNS, thus addressing the suggestion that there may be similar underlying mechanisms for stuttered and non-stuttered disfluencies ( Jonson, 1959, Postma and Kolk, 1993 and Williams et al., 1968). 1.3. Behavioral inhibition and mean length of utterance Besides stuttering, BI has been speculated to impact several other aspects of children's speech-language production. For example, Paul and Kellogg (1997) reported that clinician ratings of “Approach/Withdrawal” correlated positively with mean length of utterance (MLU) for first grade children with a history of slow expressive language development. This finding suggests that children who were more “outgoing” produce longer and more complex language during their conversational speech. To date, however, there have been few empirical studies regarding the relation between temperamental characteristics and MLU in preschool-age CWS and CWNS. Given that stuttered disfluencies as well as non-stuttered disfluencies are most apt to occur on long and complex utterances (e.g., for further review of this issue, see Zackheim & Conture, 2003), the influence of BI on MLU appears to warrant assessment due to the possibility that MLU may serve as a mediator of the association between BI and instances of stuttering. 1.4. Research hypotheses Therefore, the present study was designed to address the issue of whether behavioral inhibition is associated with childhood stuttering and speech language output of children who do and do not stutter. This investigation addressed three specific issues. First, the study addressed whether there is a difference in BI between CWS and CWNS. It was hypothesized that preschool-age CWS, when compared to preschool-age CWNS, would exhibit more behavioral inhibition as indexed by longer latency to the 6th spontaneous comment. Second, the study investigated whether more behaviorally inhibited CWS and CWNS differ in quantity of instances of stuttered and non-stuttered disfluencies when compared to less behaviorally inhibited CWS and CWNS, respectively. It was hypothesized that more behaviorally inhibited CWS would exhibit more stuttered and non-stuttered disfluencies than less behaviorally inhibited CWS during conversation. Similarly, it was hypothesized that more behaviorally inhibited CWNS, when compared to less behaviorally inhibited CWNS, would exhibit more stuttered and non-stuttered disfluencies during conversation. Finally, the study examined the relation between BI and MLU for both CWS and CWNS. It was hypothesized that more behaviorally inhibited CWS, when compared to less behaviorally inhibited CWS, would exhibit shorter MLU. Similarly, it was hypothesized that more behaviorally inhibited CWNS, when compared to less behaviorally inhibited CWNS, would exhibit shorter MLU. Overall, it was thought that findings from this empirical study would increase our understanding of possible group differences in BI between preschool-age CWS and CWNS as well as whether preschool-age children's behavioral inhibition impacts their stuttered and non-stuttered disfluencies as well as speech-language output.

In summary, preschool-age CWS, when compared to their CWNS peers, are more likely to exhibit extremely high behavioral inhibition and less likely to exhibit extremely low behavioral inhibition. Furthermore, CWS’ behavioral inhibition appears to be associated with higher frequency of stuttered disfluencies. Findings of this study provide further support for recent theories of childhood stuttering (e.g., Conture et al., 2006 and Conture and Walden, 2012) that propose that temperamental characteristics and/or related emotional processes are associated with childhood/developmental stuttering. The present findings are based on direct behavioral observation, a methodology in need of further corroboration through the use of caregiver questionnaires as well as psychophysiological measures of temperamental characteristics. Thus, present results seem to support the continued use of direct behavioral observation to study the association between young children's temperamental characteristics and stuttering, the diagnostic entity (i.e., CWS vs. CWNS), as well as stuttering, the behavior (e.g., frequency of stuttered disfluencies). By triangulating caregiver questionnaire, direct behavioral observation and psychophysiological means for investigating the association between temperament and stuttering, future researchers can improve our understanding of the role, if any, that emotion may play in the onset, development and maintenance of childhood stuttering.