More than 70 million people worldwide are thought to have some kind of stuttering speech impediment – including the current President of the United States – and experts are still continuing to learn more about the condition and what causes it.

Now a new study has revealed something that may give us a big clue into why stuttering happens and how we can treat it: When adults who stutter are on their own and think no one is listening, their stutter suddenly goes away.

And it seems to be that perception of having a listener that's key. What's important about this particular piece of research is that the study participants were convinced that no one was around to hear what they were saying, providing solid scientific evidence for how different scenarios affect the condition.

"There is a lot of anecdotal evidence that people who stutter don't stutter when talking alone, but this phenomenon has not been confirmed in the lab, mainly because it's difficult to create conditions in which people believe that they are truly alone," says Eric Jackson, a speech-language pathologist and researcher from New York University.

The researchers enlisted 23 volunteers and put them through five different scenarios: reading aloud, private speech (the only scenario where it appeared that no one was listening), repeating the private speech for two listeners, and two different conversations with researchers.

For the private speech scenario, the participants were given a trio of challenging computer coding tasks to complete, tasks known to get people talking to themselves in the past. Participants were also told that those who talked out loud while doing the task usually performed better at it.

The volunteers were falsely told that no one would be listening in while they did the computing task, though they were still being monitored and recorded by the researchers. It was the only scenario where stuttering was nearly non-existent across all 23 study participants.

stutter chartStuttering across scenarios. (Jackson et al., Journal of Fluency Disorders, 2021)

"We developed a novel method to convince participants that they are alone – that their speech wouldn't be heard by a listener – and found that adult stutterers do not stutter under these conditions," says Jackson.

Having been informed afterward that they had been deceived, all of the volunteers agreed to continue with the experiment. The next question is why the lack of an audience has such a significant effect on problems with speech fluency.

That's not something the researchers go into too much detail during this particular study, but they do note that there could be an element of feeling judged or evaluated when there are other people around to listen in.

Stuttering is thought to come about through a combination of genetics and neurophysics. One possible avenue to explore in the future is at what stage social considerations start to affect young children who stutter.

"I think this provides evidence that stuttering isn't just a 'speech' problem, but that at its core there must be a strong social component," says Jackson.

The research has been published in the Journal of Fluency Disorders.