Hesitation Disfluencies in Spontaneous Speech: The Meaning of um

Human speech is peppered with ums and uhs, among other signs of hesitation in the planning process. But are these so-called fillers (or filled pauses) intentionally uttered by speakers, or are they side-effects of difficulties in the planning process? And how do listeners respond to them? In the present paper, we review evidence concerning the production and comprehension of fillers such as um and uh, in an attempt to determine whether they can be said to be ‘words’ with ‘meanings’ that are understood by listeners. We conclude that, whereas listeners are highly sensitive to hesitation disfluencies in speech, there is little evidence to suggest that they are intentionally produced, or should be considered to be words in the conventional sense. When humans communicate, their messages are conveyed by more than just the words they use. They can, for example, use gesture to indicate what a phrase such as ‘this one’ refers to, or change their tone of voice to show that the assertion that they ‘love linguistics’ is not to be taken at face value. As well as these (presumably deliberate) additional components of communication, the words of unprepared spoken language are likely to be accompanied by a range of unintentional errors. If, for example, a speaker exchanges the onsets of two words, by perhaps saying ‘darn bore’ when intending to say ‘barn door’, it is highly unlikely that the exchange is intentional, and the occurrence of accidental speech errors like this may inform us about the nature of speech planning (e.g. Hartsuiker et al. 2005). Between these extremes of intentionality are disfluencies, or the false starts, repetitions, and hesitations that accompany the words that speakers plan and utter. Averaging across several studies, Fox Tree (1995) estimated that approximately 6% of words uttered are, or are affected by, some form of disfluency (see also Bortfeld et al. 2001). These disfluencies may not always be accidental: It has been argued that some types of disfluency should be counted among the tools the speaker has for communicating to the listener, alongside things such as tone of voice (others, such as false starts, may be due to speakers editing their own speech). Chief among the potentially communicative disfluencies are the so-called fillers, such as um 590 Martin Corley and Oliver W. Stewart  Compass 2/4 (2008): 589–602, 10.1111/j.1749-818x.2008.00068.x Journal Compilation  and uh, which (together with prolongations and pauses) mark a hesitation on the part of the speaker. In this paper, we investigate the role played by hesitation in human communication, with a particular focus on fillers and the communicative goals they may serve. Producing Hesitation Disfluencies Hesitation phenomena such as fillers are most likely to occur at the beginning of an utterance or phrase, presumably as a consequence of the greater demand on planning processes at these junctures (Maclay and Osgood 1959; Beattie 1979; Barr 2001). The view that cognitive load is an important predictor of disfluency is supported by the fact that disfluencies are found to occur more often before longer utterances (Oviatt 1995; Shriberg 1996), and when the topic is unfamiliar (Bortfeld et al. 2001; Merlo and Mansur 2004). Cognitive load is also implicated when we look at hesitations on a word-by-word basis. Investigations of where disfluencies such as fillers occur throughout utterances have established that they are more likely to occur before content words (Maclay and Osgood 1959), such as low-frequency color names (Levelt 1983). However, Beattie and Butterworth (1979) came to a different conclusion when they investigated the distributional properties of disfluencies across a set of recordings of two-person conversations. They showed that both low-frequency content words and those rated as contextually improbable were likely to be preceded by hesitations such as fillers; when frequency was held constant, contextual probability still predicted disfluency. Rather than attributing disfluency to cognitive load, Beattie and Butterworth suggested that speakers might be aware of an element of choice in selecting words with low contextual probability, and were more likely to be disfluent for this reason. Choice was also implicated in a study by Schachter et al. (1991) in which lectures in the natural sciences, social sciences, and humanities were recorded and analyzed for numbers of fillers per minute. Disfluency differed between topics, with the natural sciences resulting in the least and the humanities in the most frequent use of fillers. However, when the lecturers were interviewed on general topics, their rates of disfluency did not differ. Schachter et al. attributed the differences in lecture disfluency rates to the fact that there were fewer linguistic options in the sciences, causing lecturers to hesitate less as they selected appropriate terms. They later corroborated their claims by measuring vocabulary size in lectures, learned articles, and topic-related journalism (Schachter et al. 1994), showing that there were indeed fewer terms used in the sciences. So far, we have been speaking about cognitive load and choice as if they were different, but it is of course the case that a higher number of options to choose from could result in an increased cognitive load. In a detailed experimental investigation, Oomen and Postma (2001) manipulated speech rate, using a task modified from Levelt (1983; see also Martin