I believe that you are both right, in that, * [research ](https://pubs.asha.org/doi/epdf/10.1044/cicsd_31_S_80)mentions: "*If stuttering was completely governed by genetics, then if one identical twin stuttered, his or her twin would also stutter, and that is not the case—the rate is considerably less than 100, revealing the existence of strong environmental factors.*" * [research ](https://www.reddit.com/r/Stutter/comments/1adttz5/what_causes_stuttering_according_to_mark_onslow/)mentions: "*We know from twin studies that stuttering genotype doesn’t lead inevitably to stuttering phenotype, and that there must be environmental or other epigenetic factors which contribute to the continuation of stuttering.*"