Speech is a very tricky process for humans. It originates in the parts of the brain dedicated to language processing, but is articulated by the motor system. Using brain-imaging technology, researchers discovered that stuttering seems to be linked to a discontinuity between the two processes. More specifically, they found overactivity in certain areas of the brain and underactivity in others.
There was, for instance, underactivity in the motor system but an overabundance of dopamine [source: Watkins]. Dopamine is a neurotransmitter, or chemical messenger that allows nerve cells to communicate, that vitally contributes to movement. Too little dopamine can result in clumsiness, while too much can cause tics and repetitive, unwanted movements [source: Mandal]. The neurotransmitter plays a role in other motor-control disorders, like Parkinson's and Tourette's.
What's unclear is whether the characteristics of stutterers' brains are congenital or developmental. In other words, are people born with a stutter, or does stuttering during a crucial period of brain development cause the structural abnormalities that the researchers found?
That hasn't been answered yet, although recent genetic research discovered four genes associated with some instances of stuttering. These four genes are linked to the proteins responsible for "cellular trafficking." This means the proteins are like ushers that ensure the elements of a cell end up in the right spots within that cell. It appears that more than one neurological disorder could be linked to problems with cellular trafficking [source: NIDCD]. It could be that some children experience stuttering in a relatively common developmental stage, but a subset of that group is genetically predisposed to continue stuttering into adulthood.
So, stuttering isn't a sign of stupidity. In fact, some studies indicate that kids who recover from stuttering tend to have more advanced language skills than their nonstuttering peers. One explanation could be that kids who begin to stutter in the typical window of development simply might be developing their language skills faster than their motor skills [source: Yairi].