I participated in a study at the National Institute of Health in DC. The researcher was shocked to learn no one in my family stuttered. The researcher told me about a set of twins where one stuttered and the other did not. They were going to put me in an MRI for 1-1/2 hours and watch my brain waves as I spoke. I never made it that far as it turns out I am partially deaf in my left ear. For those who say “who cares?!” (a lot of people IRL do ask that), I assume all participants within the study should be similar to ensure validity of the study. I am no scientist but it makes sense to me.