It is really too bad when science comes down on the side of the iron fist, but the studies show over and over that justice requires peace, and peace requires trust, and diversity reduces the social capital necessary to assure trust.
On the other hand, who wants to live under ISIL or similar diversity busting regimes? But one of the first prerequisites for a peaceful society appears to be a shared culture first, at least with respect to how people treat each other. It would be nice if we could use clothing to mark culture, so that when I see someone wearing a hoodie I could KNOW that indicated a peace-monger, but it just is not so, any more than someone wearing a sash with a swastika is identifying themselves as a keeper of the word of gawd.
The question has to be, how to get social capital higher and the science appears to suggest homogeneity. I don't like that answer, any more than I liked it when simulations showed "tit-for-tat" was the preferred strategy in cooperative games. I don't like that I cannot just flap my arms and fly either, but at some level all of these findings are our realities.