Several months ago I read a fascinating and excellent book, "The Arab Mind," (Raphael Patai, published ca. 1972). Patai points this out early: the Arabs are not a race but a culture, a culture that binds them together in concentric rings of loyalty ("I against my brothers; I and my brothers against my cousins; I and my cousins against the world"), with the outermost division between the Dar-es-Salaam ("House of Peace" or Islam) and the Dar-es-Harb ("House of War", or the rest of the world?).
A very short tour of some of the topics of the book is online at: http://www.hope-of-israel.org/amind.html.
That means that Islam solidifies the mentality of nomadic tribes.