Filler Words in Japanese Speech Reveal Hidden Markers of Language Proficiency

Every language has its hesitation sounds, and Japanese is particularly rich in them. Learners sprinkle their speech with sounds like ano, eeto, maa, and sono, often without realizing how much these small vocal fillers reveal about their command of the language. A new study published in PLOS One by researcher Zuohao Chen has taken a close statistical look at these seemingly trivial utterances and found that they are far from meaningless. Far from being mere stalling devices, certain fillers turn out to be reliable signals of how advanced a speaker’s Japanese really is, while others remain stubbornly flat regardless of skill level.

The research focused on the Oral Proficiency Interview, or OPI, a widely used assessment in which trained raters assign learners a speaking level based on a recorded conversation. Rather than simply counting how often learners said um or ah, the study treated each filler type as a distinct category with its own potential function in discourse. Using a Cumulative Link Mixed Model, a statistical technique suited to ordered outcomes such as proficiency ratings, the analysis examined whether the frequency of particular filler groups predicted the OPI level a learner received. The mixed model framework also allowed the researcher to account for variation across individual speakers, a crucial step when dealing with naturally messy conversational data.

The results split the filler inventory into two strikingly different camps. Fillers from the i-group, the u-group, the ano-group, and the eeto-group showed no significant association with proficiency at all. No matter how often learners produced these sounds, their OPI ratings did not systematically rise or fall. These are the fillers that many learners acquire early and use habitually, often as automatic hesitation devices that plug pauses in speech without adding much structure to the discourse. Their ubiquity across all proficiency levels apparently renders them useless as markers of advancement.

In sharp contrast, four other groups told a very different story. The a-group, the e-group, the o-group, and the n-group fillers were significantly negatively associated with higher proficiency, meaning that learners who used them more frequently tended to receive lower OPI ratings. These short, vowel-like or nasal fillers appear to function as signs of struggle, cropping up when speakers grope for words or lose their thread mid-sentence. Their persistence at lower proficiency levels and relative disappearance at higher ones suggests they mark the disfluency that raters implicitly penalize.

The most intriguing findings concerned the maa-group and the sono-group, which were significantly positively associated with higher proficiency. Learners who used these fillers more often tended to earn higher OPI ratings, a pattern that demanded deeper explanation. Why would one pair of fillers track proficiency upward while another pair remained neutral? The answer, the study suggests, lies not in the sounds themselves but in the discourse work they perform.

To probe this question, the researcher analyzed maa and sono according to their functions and discourse roles. The analysis revealed that certain uses of these fillers require at least two clauses to be produced. A turn-internal maa, for example, typically appears when a speaker is connecting ideas across a stretch of extended talk, signaling a shift or elaboration within a multi-clause utterance. Similarly, an inferential or re-presenting use of sono involves referring back to or reasoning from something said earlier, which presupposes that the speaker has already built up enough linguistic material to reason about. In other words, these particular filler functions are only available to speakers who can sustain paragraph-level production, producing connected discourse rather than isolated sentences.

This connection to paragraph-level language production is significant because it maps directly onto one of the official criteria of OPI assessment, the Text Type dimension. Raters at higher proficiency levels explicitly evaluate whether candidates can handle extended, connected discourse, weaving clauses together into coherent paragraphs of speech. The study’s finding that fillers performing these discourse-linking functions are significantly positively associated with higher proficiency therefore makes mechanical sense: learners who can deploy maa and sono in these sophisticated ways are, almost by definition, speakers who have crossed the threshold into paragraph-level production, and raters reward exactly that ability.

By contrast, the analysis confirmed that fillers such as ano and eeto, as well as maa used within adjacency pairs, the tightly patterned question-answer or greeting-response sequences that structure everyday conversation, showed no significant increase with proficiency. These uses do not require multi-clause production. A learner can drop an ano before a single word or deploy maa in a ritualized exchange without ever constructing extended discourse. Their frequency therefore remains flat across proficiency levels, confirming that not all filler use is created equal and that the discourse role of a filler matters more than its raw frequency.

The broader implication is that fillers can be considered one of the key linguistic factors helping to explain the relationship between spontaneous speech habits and the development of oral language proficiency. For language teachers, the study hints that filler use could serve as a diagnostic window into a learner’s underlying discourse competence, revealing whether a student is stringing together clauses into paragraphs or still speaking in short, disconnected bursts. For assessment researchers, it suggests that the micro-structure of hesitation, which sounds a speaker chooses and in what discourse position, carries measurable information about proficiency that coarse counts of disfluency would miss. For learners, the message is unexpectedly encouraging: the path to sounding more proficient in Japanese may run not through eliminating hesitation altogether, but through learning to hesitate in ways that organize and connect extended thought.

Subject of Research: The relationship between Japanese filler word use and oral proficiency levels as assessed by the OPI

Article Title: The relationship between the use of fillers and Japanese proficiency: Focusing on OPI level assessment

Article References: Chen, Z. (2026). The relationship between the use of fillers and Japanese proficiency: Focusing on OPI level assessment. PLOS One, 21(10), e0360747. https://doi.org/10.1371/journal.pone.0360747

Image Credits: AI Generated

DOI: 10.1371/journal.pone.0360747

Keywords: Japanese language learning, filler words, oral proficiency interview, second language acquisition, discourse analysis, speech fluency, linguistics, language assessment, pragmatics, statistical modeling, relationship, between