One point Justice Clarence Thomas used in his concurring opinion in Fisher v. Texas was to cite the so-called “mismatch theory” of affirmative action outcomes.
Affirmative action is bad for minorities because they’re admitted to schools that aren’t really appropriate for their intellectual capabilities, so they earn lower grades. This is a common point made by affirmative action opponents, but it turns out that’s probably wrong.
As Thomas put it, under affirmative action plans, mismatch works like this:
The university admits minorities who otherwise would have attended less selective colleges where they would have been more evenly matched. But, as a result of the mismatching, many blacks and Hispanics who likely would have excelled at less elite schools are placed in a position where under-performance is all but inevitable because they are less academically prepared than the white and Asian students with whom they must compete. Setting aside the damage wreaked upon the self-confidence of these overmatched students, there is no evidence that they learn more at the university than they would have learned at other schools for which they were better prepared. Indeed, they may learn less.

But recent research suggests this really just isn’t true. According to a piece by Marc Herman at Pacific Standard :
A day after the court’s ruling, sociologists Michael Kurlaender of the University of California-Davis and Eric Grodsky of the University of Wisconsin-Madison released an analysis arguing that mismatch did not exist when students were compared to peers of similar pre-university educational backgrounds.
Kurlaender and Grodsky looked at the then-eight-campus University of California system between 2004 and 2008. The UC system is designed to admit all California students who meet a minimum high school GPA and a standardized test score. But admission to some campuses is more competitive than others. Because of budget cuts during the period studied, the system had been forced to limit class sizes at the most competitive campuses (Berkeley, the University of California-Los Angeles, and the University of California-San Diego). Several thousand were rejected as less-qualified. However, the economic situation improved, and many of the rejected students were accepted after all. In theory, this cohort was “mismatched” to the group that had not been rejected during the budget cuts—the more elite students at the most elite institutions in the system.
What they found was that, after controlling for background, the originally rejected performed just performed just as well as those originally admitted.
This doesn’t entirely undermine the mismatch theory. The study in question looked only at students admitted (without affirmative action policies) at a single university system in a single year. But it certainly complicates the story.
The researchers found that “mismatched students were at a slight disadvantage relative to their better prepared peers,” but they show that mismatch is almost entirely due to high school quality. For students who attended roughly similar high schools, there was no difference in outcome based on the point at which they were admitted.
As Kurlaender explained in a press release issued by the University of Wisconsin:
The plaintiffs in the (U.S. Supreme Court’s) Fischer case claim that the beneficiaries of affirmative action are… the victims because of mismatch. Our study shows just the opposite; mismatched students are more likely to persist in college at elite UCs and do not pay a penalty in terms of grades for doing so.