Pop Idol's Gareth Gates is a famous stammerer
The finding could lead to the development of drugs to treat the condition.
Professor Gerald Maguire of the University of California believes a stammer is like epilepsy or schizophrenia, and that is caused by a chemical imbalance in the brain.
"Our research with brain scans, which measures brain activity, shows that stuttering may be related to an area deep within the brain, the striatum, an area of the brain that may allow us to time, and properly allow, or initiate or start our speech.
"We also believe this area might be too high in dopamine, a brain chemical, in people who stutter."
Professor Maguire hopes his research will lead to drugs to treat stammering.
Research in the 1970s showed a drug called Haloperidol, used for serious brain disorders such as schizophrenia, was able to reduce symptoms in people who stammered.
Heriditary link
But the side effects prohibited its use in the majority of people with the condition.
Professor Maguire, who himself has a stammer, suggests another drug - olanzapine- which he has successfully used, could provide an effective treatment for the condition.
Norbert Lieckfeldt, of the British Stammering Association: "The problem is that no-one really knows why people stammer, though theories abound.
"There has been quite a bit of work done on brain activity, and noticeable differences have been found in people who stammer.
"The problem is that at the moment, we just don't know if the brain patterns are the cause or the effect of stammering."
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 British Stammering Association helpline can be reached on 0845 603 2001.