Stuttering typically emerges between the ages of 2 and 7 years, when the child acquires its native language. It is characterized by numerous unwanted repetitions of a part of a word, usually a speech sound or a syllable. Stutterers develop behaviours to disguise the stuttering, such as interjection of "mm" and "er" into speech, or to eliminate the stuttering, such as preparation in advance of what one is going to say, or using words that do not contain the difficult sounds.
Certain social situations worsen stuttering; some people stutter over the telephone, some when talking with strangers, some with intimate friends, and others when reading aloud.
The causes of stuttering remain unclear; the ancients assumed that tongue movement was encumbered in some way; more recently Freudian psychoanalysts have claimed that stuttering showed anal fixation, analogously to constipation. The current neurological view is that stuttering is a motor control disorder, whereby undue motor force coupled with improper processing of one's own auditory feedback disrupts one's fluency. Alternatively, inadequate use of rhythmic coordinators in the brain may be at fault, and this could either neurological or psychological causes.
55 million people worldwide stutter, of whom 80% are men.