My theory: I don't believe it's anything to to do with social pressure etc. I'm the same - I don't stammer at all if I read from a book in a room by myself. What I suspect is happening (and I think evidence supports, see below) is that the brain's wiring is just different for when you're in social situations. We're social creatures, it's not the same neural pathways when speaking in a group. That doesn't need to have anything to do with "social pressure" in the sense of nervousness etc. Example for me - I do a lot of presenting, and just recently (due to an amazing speech therapist) I'm telling people before I start: "By the way, I have a mild stammer. It comes and goes. If I get stuck, please bear with me." (Same line each time.) It turns out that's a super-power - it owns the stammer and short circuits any fall into a pit of shame I used to have when stammering (I always tried to hide it). I **do still stammer** during presentations. I just no longer care that I stammer. So - it's the social situation (that my brain can tell is happening because we're apes!) and when I'm by myself, that's not activated. Though it's not the same for everyone - I see below some who do stammer when alone. If you want a good, watchable potted history, watch the BBC's [Secrets of the Brain](https://www.bbc.co.uk/iplayer/episode/m002k781/secrets-of-the-brain-series-1-episode-1), especially episode 2 that digs into just how deeply wired our social environment and theory of other minds is. I also asked chatGPT to [gather some sources](https://chatgpt.com/share/68f791f8-e384-8013-9610-442d41ab95b8) about social wiring being different - I don't trust LLMs to get facts right, but it does OK at gathering sources and summarising them.