Men are more prone to genetic "diseases" such as a predisposition to stuttering, a direct stutter directly or color blindness for example. The genetic difference between men and women is that men have one X and one Y chromosome, whereas women have two X. The X chromosome is bigger than the Y, and happens to be more important aswell. If men have an issue with the X, they will develop a disease for example. But if a woman has an issue with one of their X, they have another one as backup. This also causes a bigger variability among men than women, I.E men are more different than women. I'm not completely sure if this applies to stuttering, but it does apply to color blindness and it would suprise me if stuttering didn't have the same mechanism.