Okay so, none of my sisters stutter, but my dad does a little bit. His stutter is a mild one, mine I'd say is moderate, borderline severe. Also I'm pretty sure some uncles I have from his side of the family stutter mildly as well, so this makes me pretty confident to say that genetics might play a big part on it. However, like I said my stutter is a lot worse than my dad's... but I did suffer from abuse when I was around 8-9, and it was at this age when my stutter started; so even though it's most likely genetic, I'd say you might need a trigger for the stutter to manifest. For me it was trauma. For other people it can be anything, but I don't really know. It's just a theory I have