I think physical and verbal abuse from parents or bullying can cause stuttering. I was pretty sensitive, shy kid. My "dad" default voice tone was yell, being agressive, emotional and hygiene neglect then started stuttering as kid. No wonders daily living in such stress house, being ashamed for not knowing xyz, being yelled for my shynes, social anxiety. No wonder such shit parenting caused stuttering. Dad/parents is toxic/narcissit abuser, I never heard from him "sorry its my fault" it always was someone else faul. He become furious if someting wasnt following his mind, like kid doing something wrong and lets yell at him, I once moved someting he got fourius but he could move anything he want. No choice in room things. Older siblings had more attention and they words were taken more seriously. He always was smartest, and classic old times argument - back then they wasnt doing it and survived. Not teaching basic life skill instead yelling for not knowing it. I remember I often cried, felt gulp in throat so I guess a lot of stress from child age hided in voice muscles. School time wasnt best honestly, I usually sat silent, because I once stuttered and got laughted by class "elite".