Fluent speech is speech that is smooth, forward-moving, unhesitant and effortless. Any break in fluent speech is called a dysfluency. Everybody experiences dysfluencies from time to time, they are for example fillers (eh, ah), hesitancies, repetitions of whole words or phrases, and revisions.
Stuttering is speech in which more dysfluencies are present than is considered average.

Stutterers experience different dysfluencies than 'normal' speakers. Characteristic for stuttering are the repetition of sounds or syllables, prolongations ( the unnatural stretching out of sounds), and blocks, where the sound seems to get stuck and can't come out.
Stuttering is different from normal dysfluency in the type, frequency and duration of the dysfluencies. The frequency can be found by counting the amount of dysfluencies in, for example, a 100 words and determining the percentage. An average speaker will have up to 7% dysfluencies in his or her speech. These dysfluencies are most often rapid and don't slow down speech. Stuttering speech contains 10% dysfluencies or more, that can last for half a second to 30 seconds. What's even more important is that stuttering is accompanied by tension in the speaker.

Most research on stuttering has been done in western countries, but the main characteristics of stuttering occur all over the world, in every language.

The onset of stuttering usually occurs around the time that language skills are developing, and the onset is generally gradual in nature. Late-onset developmental stuttering is rare. Many kids experience a period (from age 2 to 5) of nonfluency, where they repeat whole words or phrases. The words are repeated once or twice, and they are repeated easily. The child does not seem to notice the dysfluencies and also experiences no tension as a result of them. The cause of these dysfluencies probably lies in the normal development of speech patterns and speech muscles. Most children outgrow these dysfluencies, some do not.

There have been many theories about the cause of stuttering. Currently it is believed that a number of factors play a role in the development and maintenance of stuttering, that can be constitutional, environmental and communication factors.
There is some evidence that stuttering is at least partly genetic, as it seems to run in certain families. There is also evidence that stuttering is caused by a disorder in the timing of the movements of the speech muscles, a defect in auditory feedback, and a lack of cerebral dominance for language functions. Researchers in San Diego reported results of a study using Positron Emission Tomography scanning (PET scan) that supports all three of the above causes. In normal and right-handed individuals, language functions are localized in the left side of the brain. PET scanning allows one to look at brain activation during different activities. Stutterers showed a shift in brain activation from the left to the right side of the brain, suggesting that they process language differently. This activation of the right side occurred when stutterers were stuttering and when they were speaking fluently.

This means that stuttering is not an emotional disorder. Psychological factors can influence the disorder, but the basic problem is a physical one.

The stutterer's brain doesn't have good, reliable control over the complex co-ordinated muscular movements that produce speech. When things are going comfortably, when they are at ease with their listener and saying something simple and straightforward, stutterers are much more likely to be fluent than if they are talking on a difficult topic to an unfamiliar audience. The anticipation of a difficult situation where stuttering might occur and be a problem is likely to cause nervousness that can in turn worsen the stuttering.

Some facts about stuttering:

  • Stuttering occurs more often in males than in females, with a ratio of about 3:1. This ratio is even larger for adults.
  • In childhood five people in a hundred stutter. In adulthood this amount decreases to about one in two hundred.
  • Stuttering is not a symptom of emotional or mental problems. Stuttering may cause stress and emotional difficulties, though.
  • Stutterers are not less intelligent than normal speakers; they are of normal intelligence.
  • Stuttering is not learned by imitating others who stutter.
There are many different types of stutter therapy available, for children as well as adults. While therapy works best in young children, because their nervous systems are still flexible, therapy can also be succesful for adults. The goal of stutter therapy in general is not to obtain perfect fluency, which is impossible, but rather to learn to stutterer to speak as fluent as possible and, much more important, to learn to cope with the stuttering and not be embarassed by their speech impediment.