If you’ve been following the recent discussions on Language Log, then you know that there is a great deal of inter-speaker variation in the use of the fillers uh and um, despite their superficial similarity. In this post, I’ll discuss some published results, summarize some of the Language Log findings (with the obvious caveat that none of it has been subject to any sort of peer review) and explain what I think it all means for our understanding of the contrast between uh and um.
The function of uh and um
The vast majority of work on disfluencies (which include fillers like uh and um as well as repetitions, revisions, and false starts) assumes that uh and um are functionally equivalent, substitutable forms. But Clark and Fox Tree (2002) argue that they are subtly different. They claim that uh serves as signal minor delays and um signals major delays. The evidence for this is straightforward:
- Um is more often followed by a pause than uh.
- Pauses after ums tend to be longer than those occurring after uhs (though Mark has failed to replicate this in a much larger corpus, and I am inclined to defer to him).
- Um is more common than uh in utterance-initial position, the point at which speech planning demands are presumably at their greatest. 
From these results, though, it is not obvious that uh and um are qualitatively different. This has not prevented people (myself included) from making this jump. For example, Mark speculated a bit about this for the Atlantic: “People tend to use UM when they’re trying to decide what to say, and UH when they’re trying to decide how to say it.” This is plausible, but the evidence for differential functions of uh and um is lacking.
Intraspeaker differences in uh and um
The first—and probably most robust—finding, is that female speakers have a higher average um/uh ratio than males. This pattern was found in several corpora of American English available from the LDC (1 2 3). It also reported in a recent paper by Acton (2011), who looks two American English corpora. A higher um/uh ratio in females was also found in two corpora of British English. The first looks at data from the HCRC map task and the second at the conversational portion of the British National Corpus (BNC). The latter was earlier the subject of a study by Rayson et al. (1997), who found that that er (the British equivalent of uh)  was the one of the words most strongly associated with male (rather than female) speakers; the only word more “masculine” than uh was the expletive fucking.
Social class effects
The second finding is that um/uh ratio is correlated with social class: higher status speakers have a higher um/uh ratio. Once again, this was first reported by Rayson et al., who found that erm is more common in speakers with high-status occupations. Mark found a similar pattern in American English using educational attainment—rather than occupation—as a measure of social class.
The third finding is that younger speakers have a higher um/uh ratio than older speakers. This was first reported by Rayson et al. (once again, studying the conversational portion of the BNC), who found that that er is much more common in speakers over the age of 35. Similar patterns are reported by Acton, and several Language Log correspondents (1 2 3 4).
Finally, Jack Grieve looked at um/uh ratio geographically, and found that um was more common in the Midlands and the central southwest. I see two issues with this result, however. First, I don’t observe any geographic patterns in the raw data (ibid., in the comments section of that post); to my eye, the geographic patterns only emerge after aggressive smoothing; this may just be another case of Smoothers Gone Wild. Secondly, the data was taken from geocoded Twitter posts, not speech. As commenter “BK” asks: “do we have any reason to believe that writing ‘UM’ vs ‘UH’ in a tweet is at all correlated with the use of ‘UM’ vs ‘UH’ in speech?” Regrettably, I suspect the answer is no, but there still is probably something to be gleaned from tweeters’ stylistic use of these fillers.
Uh and um in children with autism
Our recent work on filler use in children with autism spectrum disorders (ASD) might provide us another way to get at the functional differentiation between uh and um. We  used a semi-structured corpus of diagnostic interviews of children ages 4-8, and find that children with ASD produce a much lower um–uh ratio than typically developing children matched for age and intelligence. Children with specific language impairment—a neurodevelopmental disorder characterized by language delays or deficits in the absence of other developmental or sensory impairments—have an um–uh ratio much closer to the typical children; this tells us it’s not about language impairment (something which is relatively common—but not specific to—children with ASD). We also find that um–uh ratio is correlated with the Communication Total Score of the Social Communication Questionnaire, a parent-reported measure of communication ability. At the very least, individuals who use more um are perceived to have better communication abilities by their parents. At best, use of um itself contributes to these perceptions.
How uh and um differ
To the sociolinguistic eye, the effects of gender, class, and age just described tell us a lot about uh and um. Given that women have a higher um/uh ratio than men, we expect that um is either the more prestigious variant, or the incoming variant, or both. This is what Labov calls the gender paradox: women consistently lead men in the use of prestige variants, and lead men in the adoption of innovative variants. Further evidence that um is the prestige variant comes from social class: higher status individuals have a higher um/uh ratio. Younger speakers have a higher um/uh ratio, suggesting that um is also the incoming variant. This is not the only possible interpretation, however; it may be that the variants are subject to age grading—meaning that speakers change their use of uh and um as they age—which does not entail that there is any change in progress. Given a change in apparent time—meaning that younger and older speakers use the variants at different rates—the only way to tell whether there is change in progress is to look at data collected at multiple time points. While the evidence is limited, it looks like both age grading and change in progress are occurring—they are not mutually exclusive, after all.
Unfortunately, some evidence from style shifting problematizes this view of um as a prestige variant. O’Connell and Kowal (2005) look at uh and um by analyzing the speech of professional TV and radio personalities interviewing Hillary Clinton. If um is the more prestigious variant, then we would expect a higher um/uh ratio in this formal context compared to the more casual styles recorded in other corpora. But in fact these experienced public speakers have a particularly low um/uh ratio. Hillary Clinton produced 640 uhs and 160 ums, for an um/uh ratio of 0.250; in contrast, Mark found that on average, female speakers in the Fisher corpora favored um more than 2-to-1.
So why is Hillary Clinton hating on um? Can an incoming variable be associated with women and the upper classes yet still avoided in formal contexts? Or are we simply wrong to think of uh and um as variants of a single variable? Is it possible that, given our limited understanding of the functional differences between uh and um, we have failed to account for associations between discourse demands and social groups (or speech styles)? Perhaps Clinton just needs uh more than we could ever know.
 This finding is so robust, it even holds in Dutch, which has very similar fillers to those of English (Swerts 1998).
 Note that, at least according to the Oxford English Dictionary, British er and erm are just orthographic variants of uh and um, respectively. That’s not to say that they’re pronounced identically, just that they are functionally equivalent.
 Early studies geared at speech researchers were conducted by Peter Heeman and Rebecca Lunsford. Other coauthors include Lindsay Olson, Alison Presmanes Hill, and Jan van Santen.
E.K. Acton. 2011. On gender differences in the distribution of um and uh. Penn Working Papers in Linguistics 17(2): 1-9.
H.H. Clark & J.E. Fox Tree. 2002. Using uh and um in spontaneous speaking. Cognition 84(1): 73-111.
D.C. O’Connell and S. Kowal. 2005. Uh and um revisited: Are they interjections for signaling delay? Journal of Psycholinguistic Research 34(6): 555-576.
P. Rayson, G. Leech, and M. Hodges. 1997. Social differentiation in the use of English vocabulary: Some analyses of the conversational component of the British National Corpus. International Journal of Corpus Linguistics 2(1): 133-152.
M. Swerts. 1998. Filled pauses as markers of discourse structure. Journal of Pragmatics 30(4): 485-496.