Stammering may be caused by irregularities in the structure of the brain.
An inability to get one's words out has long been thought to be caused by emotional factors, such as stress.
But researchers who studied adults with persistent stammering found that these individuals had anatomical irregularities in the areas of the brain that control language and speech.
The scientists used sophisticated MRI scans to measure the brains of 16 people with a condition known as persistent developmental stuttering (PDS).
These scans were compared to those taken from 16 people who did not stutter.
They found that the people with PDS had significantly larger right and left temporal lobes.
In addition, irregularities in the shape of the brain were much more prevalent among the patients with PDS.
The study also found that there were anatomical differences between men and women and right and left-handers who stammer.
Lead researcher Dr Anne Foundas, from Tulane University in New Orleans, told BBC News Online: "Our study provides the first strong evidence that anatomical variation in brain areas devoted to speech and language functions may be associated with an increased risk for the development of stuttering.
"If developmental stuttering is biologically linked and specific "biologic" subgroups are identified, then treatment protocols can be developed that target these different "types" of stuttering."
Carolyn Cheasman, a speech therapist at the City Lit Institute in London, which provides support for people who stammer, said that it was a complex problem.
She told BBC News Online: "This research may represent another piece in the jigsaw for some people who stammer.
"But for most people it is probably multi-factorial in terms of origin.
"There may be some neurophysiological factor that makes some children vulnerable, but whether or not they stammer may be determined by other factors such as what is going on in their life, and their personality."
Ms Cheasman said stammering could have a profound impact on some people's lives, but that most people could benefit from speech therapy.
About 0.9% of the UK population stammer, and three times as many males as females.
Children who stammer are normally affected from the age of between two and five, but 70% recover with little or no therapy.
Stammering is to some extent hereditary. A person with a close relative who stammers is three times more likely than the general population to stammer.
The research is published in Neurology, the scientific journal of the American Academy of Neurology.