I'm a month late to this thread, BUT The fact that your identical twin doesn't stutter doesn't automatically prove it's not caused by genetics. A mutation that leads to stuttering could still have occurred in a later stage of embryo development. Stuttering is also a 'complex genetic disorder', meaning one or both of two things: there are environmental factors in play, and/or there isn't just simply one "stutter-gene" and it's caused by interplay by many different genes. Someone here has said that it's by a majority psychological, but I don't really agree with that. Obviously there are also psychological or environmental factors that are important, since many people who stutter are 'cured' without even doing anything. I don't think some of the worst stutterers (and there really are some pretty extreme cases) would magically speak fluently if they just become more confident. If by 'psychologically' you mean 'something's physically broken in your brain', then I'd agree. It's *definitely* caused by genetics as well, because there's a 25% chance for children of people who stutter that they're going to stutter as well. I think the "genetics/environment ratio" is different for everyone who stutters, but this is something I've thought about myself and I can't back it up with evidence.