What causes stuttering?

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.