But maybe it's not just genetic, It could also be cultural(although I very much doubt it). But therapy for stutterers have shown that your attitude to your stutter can harm or improve your influency, do girls have a better attitude towards potential influencies? Probably not, but a cultural difference could be possible. More likely would be something with hormonal differences. Boys have more testosterone than girls as we all know, and the difference grows bigger with age. Maybe testosterone makes you more prone to developing a stutter? I don't know, but I would doubt this as well since most men don't have a stutter.