You have to understand what is 'root cause'. This can pindown the origin of everything that is abnormal about stutterer's abnormal speeches, of course including the diaphragm part, like when your Valsalva response is hyperactive it will also induce to diaphram's abnormal movements or whatnot, for example. If you want to pindown why you have different but abnormal presentations like stuttering for example, you have to find the 'root cause', not the 'direct cause'. Here is a crude analogy for you to understand it better: I am ugly, and my repulsive face is the direct cause of making others annoyed when they see my face, but the root cause is my genes Actually if you want to go this way deeper, as for developmental stuttering, 'stuttering predisposition' which is led by your genes is the root cause but so far you can't edit your genes so we shoud give up researching how genes are influencing our speeches thereof. We need to pindown the root cause in our brains and try all means to fix it, instead of that in genes.