The 80-85% number comes from twin studies. So basically this means that if one twin stutters, the other is 80-85% more likely to stutter due to genetics.. What's interesting is that in my family my dad stutters, my brother and sister don't. But nobody in my dads family stutters.... genetics is crazy. Personally, I am glad that genetics has such a high involvement because we can learn about them and create a path forward, it means that there's hope.