My 4 brothers and I all stuttered as children. The oldest 3 of them grew out of it, but me and the brother closest to my own age didn't. I could attribute this to just about anything. Maybe we got a lower standard of public school speech therapy than the other 3. Maybe we heard the other 3 stutter for so long that this somehow influenced our linguistic processing. Maybe our home life was worse at a critical stage for us. Maybe our parents slacked when our time came, as we were the youngest, and weren't as vigilant with us about language. Maybe it's just random genetics or chance. Either way, all variables are out of our control.