According to [Johnson and Brown](https://www.google.com/search?q=%22research%22+%22stuttering%22+what+letter+stutter+on+the+most) (researchers): >In one of the early pertinent studies, Johnson and Brown (1935) analyzed more than 300,000 words in the speech of 32 adults who stuttered. Looking at the distribution of stuttering, they found that the most frequently stuttered speech sounds were /z, l, j, t, and g/.