Stuttering, which affects 5 percent of children, usually begins between ages 2 and 6. About 50 percent of stutterers have family members who have stuttered.
Scientists are now starting to untangle the complicated issues of stuttering with the hope of finding a better treatment.
Researchers say stuttering is really a speech production problem, dispelling longstanding misconceptions that the underlying causes are language problems or psychological problems like anxiety or trauma.
Speaking involves brain area responsible not only for language, but for hearing, planning, emotion, breathing and movement of the jaw, lips, tongue and neck.
“People who have stutter have motor difficulties in producing fluent speech,” said Luc De Nil, a speech language pathologist at the University of Toronto.
“They don’t have difficulty developing words or syntax, although they may process language differently. They have difficulty with efficient coordination of motor movements and speech is such a high demand fine motor skill that requires extremely fast sequencing and timing.”
Anne Smith, a stuttering expert at Purdue University, said that in stutterers, “the generals in the brain, who control soldiers, which are the muscles, aren’t sending out the right signals to the soldiers, so they just get all mixed up and run around.”
“We really have not been able to find indicators of stuttering before the first day it emerges,” said Nan Ratner, an expert at the University of Maryland.
The late start contributed to myths that bad parenting could cause stuttering, she said. It usually begins “when you’re starting with the grammar of the language.”
The complexity of grammar, in fact, seems to be a part of the problem. Stutterers’ brains respond to meaning errors as normal speakers’ brains do, but have a much lower response to grammatical errors, said Dr. Smith.
About 75 percent of children eventually stop stuttering some with therapy and some without, but there is no predicting who will recover.
So far, drugs have shown unimpressive results, or caused severe side effects, and experts say the problem is so complex that a single pill is unlikely to cure all stuttering.
But scientists are examining images of the brains of people who began stuttering as children and those who started after a stroke, Dr. De Nil has found excess activity in areas involved in speech motor control and coordination of the movements needed for speech.
These brain areas may be working overtime because stutterers do not develop the “automatic pattern of speaking” that non stutters have, said Dr. Smith.
Only about a third of stuttering children have other language or speech problems, but Dr. Ratner found that while stutterers perform within normal limits on standardized tests, on average their scores are lower than non stutterers.
Genes almost certainly play a role for about half of all stutterers: environmental factors may contribute too. Trauma and stress do not cause stuttering but it can exacerbate it.
For unclear reasons, boys are likely to stutter, and up to 4 times as likely to continue into adulthood.