Mental and physiological, mostly. I believe social elements are of a secondary importance, given people are the trigger to the stutter, not the cause. As it stands, there are three "recognized" types of stutter: developmental, psychogenic and neurogenic. Barring only blunt trauma and injury (and potentially birth defects) to the brain, I do not see how you can differentiate stutterers into one specific category. I see stutter as much deeply interconnected with each category. The neuropsychological literature on stuttering deems it as a structural issue in brain function. Most of us are fluent by ourselves, so I find that explanation to be bogus. All research on stutterers is a sort of illustration of Schrodinger's dilemma. If a stutterer is aware he/she is being observed, their performance on cognitive tasks declines, and so researchers deem structural issue. I believe most of us would score similarly to fluent control groups if we were unaware we were being observed. Ultimately, I'm more and more convinced stuttering is habitual by nature in stutterers that are fluent when alone. The crux of stuttering research and our personal take on it focuses on the anticipation or moment of stutter as the spotlight to be studied and combated against. The reason we do that is understandable. When we're alone, we are fluent, so we can't access the "realms" that potentially need studying. I'm focused on developing ways of physiologically and psychologically priming ourselves for fluency, but not through directly exercising speech. It's our alone time when we must introduce newer long-lasting habits which translate into fluency, but not limited to. I believe speech therapy is a useful tool in the absence of a better one, but not the ultimate tool. If I'm fluent by myself, I don't need to relearn pronunciation and intonation of words and sentences. That's a superficial fix, much as building a roof in a house with broken windows. The rain will seep in anyway, just less than otherwise. It also leads to artificial and monotonic expression. We all have the right to speak our hearts out at our rhythm so we can express the fullest of ourselves.