Investigating two phonological subtypes of stuttering in Japanese adults: A vowel-consonant perspective

Stuttering is influenced by various linguistic factors, one of which is whether a word begins with a vowel or a consonant. In many languages, including English, consonant-initial words are more likely to elicit stuttering, and individuals who stutter more frequently on vowel-initial words are relatively rare. However, in Japanese, individuals who tend to stutter more on vowel-initial words are frequently observed. This study aimed to clarify these inconsistencies by exploring whether subtypes exist among Japanese speakers who stutter, characterized by vowel- or consonant-oriented difficulty. Fifteen adults who self-identified as people who stutter each read 190 nonword stimuli controlled for word length and familiarity, rating their stuttering severity on a 4-point scale after each trial. A hierarchical cluster analysis of stuttering severity across 19 different word-initial sounds revealed two distinct clusters: one primarily prone to consonant-initial stuttering (Cluster C) and another prone to vowel-initial stuttering (Cluster V). Subsequently, a multilevel ordinal logistic regression model examined the effects of various phonological features. Nasal consonants were associated with reduced stuttering severity in both clusters, whereas features such as consonantal and glottal factors were linked to increased severity only in Cluster C. These results suggest that dividing participants into vowel- and consonant-prone subtypes can reconcile previous contradictory findings regarding the role of initial sounds in Japanese stuttering. Recognizing these subtypes may be critical for future research and clinical interventions, as it underscores the need to consider individualized phonological sensitivities when investigating and treating stuttering in Japanese and potentially other languages.