I think it's both. You can look at stuff like the [monster study](https://youtu.be/h5yPwWybgD4?si=bN3pe7GzA5aSxjwv) it presents as psychological I think a lot of it is our self-confidence and the ease with which we handle stuttering. I say both because when even today, my fluency is greatly improved i still occasionally stutter over blocks and such. But i resolve it through physical means, by doing my breath trick. (basically, if i start to stutter over a block i stop talking. Forcing it out never works. I take a breath in and on the exhale I try again. The exhale breath automatically relaxes all of those cumbersome muscles in our throats and vocal cords) it works for me