Nunes (1994) proposes a way of reconciling psychological and physical aspects of stammering by a defect with respect to a buffer, already proposed by other authors.
Such a defect helps to explain five things.
- Stammering is only attested when language is already well-established, most often in early childhood, sometimes later, but never on first words.
- Like most disorders of speech and language, stammering affects more boys than girls, and often runs in families.
- The fluency of most stammerers is improved by hearing themselves speak over a delay, known as ‘Delayed Auditory Feedback’ or DAF; optimally around a third of a second, in contrast to most normally fluent speakers, who block violently and exhibit a variety of the characteristic manifestations of stammering. This has been tested and confirmed in numerous studies of fluent speakers and stammerers. The scale of the effect varies across individuals. The timing is clearly hard-wired and universal across languages and cultures.
- There is the equivalent of stammering amongst native users of American Sign Language, albeit at only one tenth of the rate in speakers. (America is the only country with enough native signers and other demographic information to get reliable statistics about a phenomenon which only occurs at a rate of one or two per thousand signers).
- There is such a thing as what is known as ‘covert stammering’ where the speaker has the characteristic self-awareness of stammering, but without displaying any of the characteristic behaviours such as ‘blocking’.
Updating Nunes (1994), one utility of such a buffer is the process evidenced in language after language, allowing words like who, what, where, to be correctly understood when they are pronounced at the beginning of sentences like “Where do you think they said I might have put the car keys?” and understood at the opposite end after any number of intervening clauses. This does not happen in all languages. But even in languages in which the Wh element seemingly stays put, there is evidence of a shifting which is just not pronounced.
A third of a second is just enough for a syllable.
If speech and language have evolved, the buffer is a supportive adaptation, storing a small amount of shifted material until it can be interpreted.