Stuttering is a speech disorder with involuntary repetition and prolongation of sounds and syllables, and silent blocks in which the stutterer’s vocal folds close and prevent the release of air and the production of sound. These obstacles negatively affect the pace, clarity, and prosody (emotional content) of speech. Stuttering includes abnormally high activity in the speech production muscles, specifically respiration, vocal folds, and articulation (lips, jaw, and tongue).
Stuttering typically occurs on the first sounds of words and the first words of sentences. Stuttering typically occurs on longer words. Adults typically stutter on content or pragmatic words (typically nouns, verbs, adjectives, and adverbs) that carry more meaning than function words such as pronouns, prepositions, or conjunctions. Stuttering also tends to occur on less frequently used words. 
Stuttering also often includes struggle behavior, in which the stutterer tries harder to speak, increases his speech-production muscle tension, and stutters more severely.
How Fluent Speech Is Produced
Speech begins with breathing, also called respiration. Your lungs fill with air, more air than you would inhale if you weren’t talking. You expand your upper chest and your diaphragm (belly) to get all this air in. Your lung pressure and respiration muscle tension increase.
Next, you release air through your throat, past your vocal folds (also called vocal cords). Your vocal folds are a pair of small muscles in your larynx. If you tense these muscles slightly, and release a little air, your vocal folds vibrate. This is called phonation. It’s also called the fundamental frequency of your voice. If you place your fingers on the front of your throat, then hum or talk, you can feel your vocal folds vibrating.
Adult men vibrate their vocal folds about 125 Hz (125 times per second). Women vibrate their vocal folds about 200 Hz. Children’s voices are even higher. This is too fast for your brain to control. Vocal fold vibration is the only muscle activity that your brain doesn’t directly control. Instead, phonation results from the coordination of respiration muscles to release air with slight tensing of your vocal fold muscles.
The key word in that last sentence was coordination. Stuttering is largely a disorder of poorly coordinated speech production muscles.
If you tense your vocal folds too much, you block off your throat and stop air from escaping your lungs. This is a good when lifting heavy weights. By blocking your larynx and tensing your respiration muscles, you increase lung pressure, which strengthens your chest and you can lift more weight. Similarly, tires inflated to high pressure can carry a heavier car. But that’s what stutterers do when they talk, and it’s not a good idea.
The space in your throat above your larynx is called the pharynx. Above your pharynx are your oral and nasal cavities. These spaces create vocal resonation. This is like the echoing of a cathedral or tunnel. The unique shape of these spaces makes each of our voices sound unique.
Your jaw and lips and tongue, collectively called the articulation muscles, modify your voice into intelligible speech.
Vowels and voiced consonants (such as /b/ and /d/) are produced by your vocal folds, and modified by your articulation muscles (jaw, lips, tongue).
Other consonants are voiceless, such as /p/ and /t/, produced by your articulation muscles modifying airflow, without your vocal folds vibrating. When you whisper, you don’t vibrate your vocal folds. You just modify airflow with your articulation muscles.
Speech requires coordination of over 100 muscles. The average person speaks about 150 words per minute. Each word requires a different configuration of most of those muscles. Speech is our most complex neuromuscular activity.
Core Stuttering Behaviors
- Disordered breathing, including antagonism between abdominal (belly) and thoracic (upper chest) respiratory muscles; complete cessation of breathing, and interrupting exhalation with inhalation.
- Disordered vocal folds, including high levels of muscle activity or muscle tension; poor laryngeal muscle timing, such as starting phonation too late or holding tension too long; and poor coordination of laryngeal muscles, e.g., incompatible contractions of opposing muscles.
- Disordered articulation, including dysfunctions of the lips, jaw, and tongue in stuttering. In general, stutterers place their articulators in the right positions (in contrast to other speech disorders such as lisping), but time the movements wrong.
Secondary Stuttering Behaviors
Secondary stuttering behaviors are unrelated to speech production:
- Gross (large) muscle movements such as eye-blinking, head jerks, body spasms, gasping, clenching of teeth or fists.
- Fear of certain words or sounds, avoidance of feared words, substitution of another word, or postponement of a feared word by adding pauses or filler words.
- Interjected "starter" sounds and words, such as "um," "ah," "you know," or "in other words."
- Repeating a sentence or phrase "to get a running start."
- Vocal abnormalities to prevent stuttering, such as speaking in a rapid monotone, affecting an accent, or using odd inflections.
- Looking away, not maintaining eye contact.
- Articulating an unrelated sound, e.g., forming a /t/ sound when trying to say /s/.
- Low-frequency tremors in the neck, jaw, and lip muscles of adult stutterers. These are found to a lesser extent in older children, and not found in young children who stutter. 
The stutterer initiates secondary behaviors to distract his attention from his speech-production muscles, leading to his speech-production muscles relaxing and producing the intended word. However, the secondary behaviors soon become part of stuttering. I remember my mother telling me that if I could just stop the head jerks and body spasms my stuttering would be more tolerable to listeners, but I had no more control of my secondary behaviors than I had of the core stuttering behaviors.
Incidence and Prevalence
About 2.5% of preschool children stutter now (prevalence).  The incidence of preschool stuttering is about 5%. In other words, about one in twenty of children stutter at some point in childhood.
Less than 1% of adults stutter. 0.73%, or about one in 135 adults, was the figure found in a one study.  That suggests that about two million Americans stutter. But I’m skeptical of that figure:
- The largest stuttering therapy program, the National Center for Stuttering, has treated about 10,000 stutterers. The second-largest stuttering therapy program, the Hollins Communication Research Institute, has treated about 5,000 stutterers. 
- Less than 400 speech-language pathologists are board-certified Fluency Specialists. Most treat only a handful of adult stutterers each year.
- About 2500 Edinburgh Masker anti-stuttering devices were sold in the United States in the 1980s after two national television appearances. SpeechEasy sold about the same number of anti-stuttering devices after appearances on Good Morning America, Oprah, and other major media outlets.
- The National Stuttering Association has about two thousand members.
- The biggest English-language stuttering support e-mail list has about three thousand members.
How many stutterers have you met, out of thousands of people you hear talking every year? I suspect that the number of adult stutterers in the United States may be in the thousands, not in the millions.
 Kalinowski, J., & Saltuklaroglu, T. Stuttering. Plural Publishing (ISBN 1-59756-01101; 2007), pages 37-39.
 Kelly, E. (1995). Orofacial Muscle Activity of Children Who Stutter. Journal of Speech and Hearing Research, 38.
 Adele Proctor, Ehud Yairi, Melissa C. Duff, & Jie Zhang. (2008). Prevalence of Stuttering in African American Preschoolers. J Speech Lang Hear Res, 51, 1465-1479.
 Craig. A, Hancock K, Tran. Y, Craig. M, & Peters, K. (2002). Epidemiology of stuttering in the communication across the entire life span. Journal of Speech Language Hearing Research, 45:1097-1105.