Part of Noam Chomsky's famous revolution in linguistics (and cognitive science more broadly) was to focus on linguistic competency rather than performance. People stutter, use the wrong word, forget what they planned to say, change ideas mid-sentence and occasionally make grammatical errors. Chomsky focused not on what people do say, but on what they would say without any such slip-ups.*
This certainly simplified the study of language, but one has to wonder what this spherical cow leaves out. Economists similarly made great strides by assuming all people are perfectly rational, think on the margin, and have full access to all necessary information free of cost. However, any theory based on these clearly false premises is limited in its explanatory power.
Speech errors carry information. This was brought home to me by a recent email I received which began, "Er, yes." If filler words carried no information, why transcribe them? (Lancelot once asked a similar question.) However, people clearly do. A quick Google search found over seven million hits for "uhhh" and over twenty-one million hits for "ummm." These include quotes like "Ummm... Go Twins?" and "Uhhh... What did she just say?"
These two quotes are suggestive, but I don't know if all transcription of filler words and other speech errors can be explained as a single phenomenon. I did hear of one study where listeners normally assume that if someone pauses and appears to have difficulty finding a particular word, the listeners assume the word is low-frequency. However, listeners drop this assumption if they believe the speaker has a neurological impairment that affects speech.
I expect that many phenomena dismissed as "performance" rather than "competence" are in fact important in communication. Whether one believes that communication should be part of any theory of language is debated (Chomsky seems to think language has nothing to do with communication).
*This part of linguistics is still very influential in psychology. I'm not sufficiently current in linguistics to say whether most linguists still do research this way.
20 hours ago in Variety of Life