First, stuttering can be passed genetically down in families. It's pretty common. For example, my mom, my aunt, and my maternal grandmother all stuttered. This has been well documented. Not all cases of stuttering are hereditary, however. IIRC, it is 3 times more likely to have a legit hereditary component. It can, in some cases, be learned, or even taught. There was this case study many years ago with orphans where they convinced these kids that they did stutter when they, in fact, did not. Over time these kids got convinced, by the people running the study, that they did stutter. Every time they didn't speak perfectly thy were told it was the stutter. These kids, over time, developed real stutters. The people running the test therefore called it a success. This is now known as the "monster study" and these people are being flogged with whips by the agents of chthulu.