Its both! There is a physiological component plus a psychological one. Research shows differences in the speech areas of the brain, (Broca’s and Werniki’s areas, neurotransmitter uptake, etc. There's even a relationship between the brains of people who suffer from both Turette’s disease and stuttering. Then there are all the psychological and social aspects that can trigger the particular unique variation in each person. It might be that the physiology potentiates stuttering in individuals with some latent psychological conditions. Its still a mystery that needs investigation. A very useful r/Redditor posts science-based information. Check out: Little_Acanthaceae87 on r/stutter. Good luck