The short answer is the cause of stuttering is part genetic and part environmental. You can already by pre-disposed to to stutter and something in your environment act as a trigger. Or you can have a heavy pre-disposition, and start stuttering as soon as you begin speaking, which is my case. A lot of people on my dad's side, starting from my grandmother, stutterers. For a more in-depth answer from someone more well read on the subject with research based evidence, you can check these videos: [https://www.youtube.com/watch?v=y3iSzlUms3s](https://www.youtube.com/watch?v=y3iSzlUms3s) [https://www.youtube.com/watch?v=XhZ0654d2WU](https://www.youtube.com/watch?v=XhZ0654d2WU)