More than 80 million people worldwide, including President Joe Biden and singer-songwriter Ed Sheeran, experience stuttering, a condition that affects the fluency and timing of speech. People who stutter may repeat sounds, syllables or words; may prolong sounds; and may experience interruptions or blocks in speech. Stuttering can interfere with a person's daily life, by triggering anxiety around social situations, for example.
Despite the high prevalence of stuttering, scientists still don't know a lot about what causes it — at least in most cases. There's not enough evidence to definitively explain the neurobiology of stuttering, Elina Tripoliti, a clinical specialist speech and language therapist at University College London, told Live Science. The current consensus among researchers is that the condition is likely caused by a mixture of factors, including genetics, differences in brain structure and function, and a person's environment.
Rare, acquired forms of stuttering can occur after a brain injury with a specific cause, such as a stroke or Parkinson's disease.
However, most people who stutter have what is known as developmental stuttering, which first arises in childhood, around ages 2 to 5, but disappears in up to 90% of children before adulthood. This form of stuttering has proved a lot harder to explain, although several hypotheses have been suggested over the years.
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One major cause of developmental stuttering may be genetic. Developmental stuttering often runs in families, with studies of identical twins suggesting that genetics may be responsible for more than 80% of cases of the condition. Several small studies have gone one step further and pinpointed specific genes that may play a role in stuttering. These include genes that are involved in the movement of materials within cells or in regulating the transmission of the chemical dopamine in the brain.
Men are around four times more likely to stutter than women. Scientists don't yet know why this is the case, although some theories exist. For example, women may somehow be more resistant to inheriting a stutter than men.
There are also subtle differences in the brains of people who stutter versus those who don't. These differences are located in regions of the brain that are responsible for planning and executing speech, including the inferior frontal gyrus and the left motor cortex.
In addition, a 2024 study revealed that stuttering may be triggered by disruption to a network of neurons in the brain that links the amygdala, the putamen and the claustrum. These three parts of the brain are respectively involved in regulating emotions, controlling movements and relaying information between different parts of the brain.
Environmental factors can also influence stuttering. Negative reactions from family or close friends can create anxiety around a person's stutter that worsens their symptoms, Tripoliti said.
Over the years, stuttering research has been limited because scientists can't study it in animals as they would other conditions, she said. Without this knowledge, it may be hard to develop targeted treatments.
Several studies have investigated the use of drugs to reduce stuttering, but there is currently nothing on the market that can eliminate it, Scott Yaruss, a professor of communicative sciences and disorders at Michigan State University, told Live Science.
"The medications that have been studied reduce stuttering for some people, some of the time," he said. "That is OK; there's nothing wrong with that, but it's not a cure."
Therapy can help some people speak more easily and accept that they sometimes stutter, Yaruss said. Encouraging society to embrace stuttering as a form of verbal diversity, rather than viewing it as a condition that must always be "treated," is also important, he added.
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