Why do colleges have diversity plans? It may seem an obvious question. After all, most colleges have diversity plans and frequently quote from them. When there is an ugly racial incident on campus, colleges administrators are quick to say how inconsistent it is with their diversity and inclusion policies. When colleges unveil a new recruiting plan, their leaders talk about how it is consistent with their stated values and hiring policies and practices. But why do the policies exist? A paper published Monday in Proceedings of the National Academy of Sciences says that most colleges "assert that diversity provides compelling educational benefits and is thus instrumentally useful." That's not surprising, because several decisions of the U.S. Supreme Court have given that as a reasonable reason -- within certain constraints -- for having an affirmative action plan.