Affirmative action also creates distortions in the pipeline for new hirees.  For example, since essentially all private elite universities in the United States systematically discriminate on the basis of race, for a given "quality" of applicant, minority applicants are likely to have a more prestigious degree.  This tends to concentrate the top-of-the-field scholars MIT wants to attract at a very, small set of institutions, since if you move just a little down the institutional prestige scale you'll find schools happily admitting minority students who are not cut out for careers at world-class research institutions.<p>This is difficult to verify because universities lie pervasively about their systematic racial discrimination, but when data does come to light about them, for example in the Gratz v Bollinger litigation, it typically shows that the discrimination is less a "plus factor" and more "two institutions at different academic tiers which happen to be geographically co-located".  (The difference between the mean white and black student at the University of Michigan is more than the difference between the mean UoM student and the mean Harvard student.)