Researchers have pushed the science of stuttering a long way past the days when it was considered a product of poor parenting or a sign of nervousness. Genes gone awry and brains working in overdrive are now considered the likely culprits behind it.

From studies on hand-clapping in children to stuttering mice, scientists at the AAAS Annual Meeting have approached the puzzle  from different directions. They're hoping to soon converge on a detailed biological model that will explain why stuttering starts when it does, why some people are more susceptible, and what sorts of therapies are the best at treating the disorder.

Stuttering usually appears between the ages of 2 and 4, said geneticist Dennis Drayna, "and half of all people who present themselves for stuttering therapy have a family history of the disease."

The familial link convinced Drayna, a geneticist at the National Institutes of Health, to screen people who stammer for possible stuttering genes. He and his colleagues have now identified a handful of genes, mutated in stuttering adults, that are involved in cell metabolism. These mutated genes may affect brain cells, he suggests, "that are exquisitely sensitive to relatively mild metabolic abnormalities."

Mice designed to carry the mutated genes could help the scientists figure out what the link between gene and behavior might be; the genes are expected to disrupt a mouse's usual  vocalizations, but Drayna acknowledged that so far it's hard to hear and assess those sounds.

Lu De Nil, a speech disorder researcher at the University of Toronto, said "the brain actually looks different in people who stutter." Using techniques such as functional magnetic resonance imaging, De Nil has shown that people who stutter often display "overactivity" in parts of the brain related to motor control. Intriguingly, people who start stuttering after a stroke sometimes have lesions in these same areas of the brain, he noted.

Anne Smith is looking for the early markers of chronic stuttering in 4- and 5-year old children at her lab at Purdue University. Children who stutter at this age sometimes find it difficult to clap to a beat, she found, and move their mouths and lips differently from children without a stutter when they talk.

She hopes that these tests and others will reveal why nearly 75% of children who start out stuttering lose the stammer spontaneously or through therapy. The severity of a child's stutter doesn't predict whether the stutter will disappear, she noted, so researchers are looking for other physical signs to predict recovery.

Evidence from biology makes it clear that stuttering doesn't come from a lack of confidence, and should be treated as early as possible, Smith said.

Jennifer McGuire, a 30-year-old who has stuttered since she was 3, joined the experts at a news conference at the meeting to say she is excited to see so many new avenues of research about the disorder.

Now pregnant, McGuire said: "It would be great if my child ends up being a person who stutters, but is able to have it be less mysterious, and have access to effective therapies at an earlier stage of life."