I also stutter less in my second language (English). I have been thinking of it for a while. Let me show you my thoughts: 1. English is more rhytmical than any romance language. People who stutter don't do it when they sing, so it makes sense that you would stutter less in a more rhythmical language. 2. English is a more economical language than Spanish (I think there's an appropiate definition for this, but I forgot). It means that you can say more with less words. Consequently, you don't use so many long sentences and there's less chance you'll stutter. 3. Maybe the craziest one, but there has been many experiments which prove that when you speak a different language your whole personality changes. But, in my experience sooner or later your native personality will meddle with your second language personality. So, ironically it's probable that your wife's brother will stuttera little bit in English as he becomes more fluent. At least that happened in my case. EDIT: I was trying to learn french, but I quit because I couldn't even say a fucking word. Right now I'm giving it a shot to Portuguese and even though I stutter more I like it.