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.
55 million people worldwide stutter, of whom 80% are men.