My mum, uncle and great grandfather stuttered. It's definitely genetic, combined with a lot of stress in early childhood (I spent my very early childhood in Russia during the 90s, enough said lmao). On the other hand, my little sister, obviously has similar genetic background but she was born in Australia and had a very calm, stable childhood. She's perfectly fluent. My non-expert theory is that stuttering is a combination of genetics and some form of childhood stress or trauma.