I realize this comment is two years old at this point but.. this is just something I’ve thought of. Maybe people who are slightly more intelligent exhibit stuttering because they are likely to use the default network mode, (DMN), of their brain more often of the time. Which is known to be active when you’re not grounded in reality, or not focused on what is happening around you. I would think DMN plays a significant role in forward thinking, or forethought, but can cause stuttering when it becomes overactive since its overuse could become a mental habit (especially for people who stutter and experience maladaptive daydreaming). Maybe when someone is too comfortable spending most of their time in forward thinking, it can start to become uncomfortable to completely focus on verbal communication. DMN is also active with people who have high levels of self-awareness, which from what I’ve heard, is a prevalent factor in mental illnesses such as depression. Again, I am probably wrong about this, there are likely details I’m missing or factors I’m not taking into account, I could be wrong. I am mostly basing this off of my personal experience with stuttering and I generally just see this as an interesting connection to think about.