… is from page 32 of Thomas Sowell’s excellent 1984 book, Civil Rights: Rhetoric or Reality?  (original emphasis):
It would perhaps be easier to find an inverse correlation between political activity and economic success than a direct correlation. Groups that have the skills for other things seldom concentrate on politics. Moreover, politics has special disadvantages for ethnic minority groups, however much it may benefit individual ethnic leaders. Public displays of ethnic solidarity and/or chauvinism are the life blood of ethnic politics. Yet chauvinism almost invariably provokes counter-chauvinism.