It's both, some neurological diseases like MS, Dementia, Parkinsons, Stroke, etc..can cause stuttering and some Psychological conditions like PTSD and Chronic Anxiety can cause stuttering. If your brain has no damage, scars, or lesions and looks normal than it's probably psychological and something that can be improved with over time. If it's neurological and you have brain damage that effects your speech then that's a different story.