Is Stammering a mental/psychological or physical issue?

Is Stammering a mental/psychological or physical issue? Hi guys. Do you all think Stammering is a mental/psychological or physical issue? I personally think it’s all purely psychological. I know I can speak perfectly fluent whilst by myself, and often with family. I can record myself speaking fluently whilst reading out loud. I tend to struggle the most with authority figures/meeting new people. This makes me think it’s not a physical issue at all, it’s a habit I’ve created. I do think it’s a confidence issue more than anything. I often get nervous/anxious in uncomfortable situations and this produces the blocks. Interested to hear your thoughts.