It’s 99% psychological for me. (Stutterer for 25 years) I can speak fluently out loud to myself and read fluently (mostly) out loud to myself. With others, I can’t do either. Therefore it’s in my head. The remaining physical aspects are symptoms, such as breathing, sweating, ticks, facial expressions, etc. The emotional aspects are also symptoms that have been reinforced by stuttering, such as anxiety, fear, social isolation, etc. I’m not sure where else it could be coming from.