Richard Sander and Stuart Taylor examine a paradox of affirmative action:

Large preferences often place students in environments where they can neither learn nor compete effectively – even though these same students would thrive had they gone to less competitive but still quite good schools.

We refer to this problem as "mismatch," a word that largely explains why, even though blacks are more likely to enter college than are whites with similar backgrounds, they will usually get much lower grades, rank toward the bottom of the class, and far more often drop out. Because of mismatch, racial preference policies often stigmatize minorities, reinforce pernicious stereotypes, and undermine the self-confidence of beneficiaries, rather than creating the diverse racial utopias so often advertised in college campus brochures.

UCLA's experience offers an example of the mismatch effect. When California Prop 209 banned racial preferences in the 90s, UCLA saw "a 50 percent drop in black freshman enrollment and a 25 percent drop for Hispanics":

Throughout these crises, university administrators constantly fed agitation against the preference ban by emphasizing the drop in undergraduate minority admissions. Never did the university point out one overwhelming fact: The total number of black and Hispanic students receiving bachelor's degrees were the same for the five classes after Prop 209 as for the five classes before.