It’s both. It’s neurological with anxiety and confidence playing a large role. People who have terrible trauma don’t usually stutter and people who stutter don’t necessarily have trauma. It’s a predisposition to stuttering (mild for some, severe for others;) that is heavily influenced by psychological factors. For some it is more the psychological, for others it is more the neurological. I know a man who had an abusive stepfather as a child. when the stepfather moved away his stuttering gradually got better. So his predisposition to stuttering was a much smaller aspect than the psychological. For someone with a severe stutter who doesn’t show much variability in stuttering in high stress vs low stress situations the predisposition is the main element with psychological element much less significant.