https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8629878/ Below is copy and paste from paper: “Adults Who Stutter Do Not Stutter During Private Speech” Abstract “Adults who stutter tend not to stutter when they are alone. This phenomenon is difficult to study because it is difficult to know whether participants perceive that they are truly alone and not being heard or observed. This may explain the presence of stuttering during previous studies in which stutterers spoke while they were alone. We addressed this issue by developing a paradigm that elicited private speech, or overt speech meant only for the speaker. We tested the hypothesis that adults do not stutter during private speech.”