New data on Affirmative Action:

There is now increasing evidence that students who receive large preferences of any kind—whether based on race, athletic ability, alumni connections or other considerations—experience some clear negative effects: Students end up with poor grades (usually in the bottom fifth of their class), lower graduation rates, extremely high attrition rates from science and engineering majors, substantial self-segregation on campus, lower self-esteem and far greater difficulty passing licensing tests (such as bar exams for lawyers)...

Research on law schools by one of us (Richard Sander)—hotly disputed by some scholars when it was published in 2005 by the Stanford Law Review and now confirmed by economist Doug Williams—found that mismatch essentially doubled the rate at which blacks and Hispanics failed bar exams. Under existing preference policies, only one in three blacks entering law school graduates and passes the bar on his or her first attempt (compared with two in three whites). Simply by reducing mismatch, we could get this ratio up to one in two.

This same dynamic turned up when several leading educators wanted to find out why so few black students went on to become professors. Funded by the Council of Ivy League Presidents, sociologists Stephen Cole and the late Elinor Barber surveyed thousands of young African-American students entering a broad cross-section of selective schools. The 2003 Cole-Barber book, "Increasing Faculty Diversity," concluded that large racial preferences and the ensuing mismatch led directly to lower grades and diminished intellectual self-confidence. They found that promising young black students who wanted to become professors abandoned their academic aspirations in droves, while similar black students who weren't mismatched were far more likely to stay the course.

What about improving diversity?

Economics professor Peter Arcidiacono and his colleagues at Duke University found in a 2011 study that students were much more likely to become friends with classmates they saw as academically similar to themselves. Students with large preferences were more likely to self-segregate and find themselves socially isolated.

The reason wasn't racism. At Duke University, for example, large numbers of whites and blacks formed friendships at the outset of college. But for those with large academic gaps, the friendships atrophied. Using their multischool results, Mr. Arcidiacono and his colleagues concluded that smaller preferences at the most selective schools would tend to increase both the likelihood and the number of cross-racial friendships at elite schools in general, despite declines in the numbers of black and Hispanic students at the most elite schools.

What about helping the most disadvantaged?

In the 1970s, when racial-preference programs were getting off the ground, universities went to great lengths to admit black students who were the first in their families to attend college. A majority of blacks attending selective schools in 1972 came from families in the bottom half of the socioeconomic distribution. Over time, however, complacency and the rapid rise of the black upper middle class has changed that; in the 1990s, only 8% of black students at selective schools came from the "bottom half," according to data from the National Educational Longitudinal Study.

Even as social scientists have transformed our understanding of affirmative action, universities don't seem to be paying attention. Consider the University of California system, which since 1998 has been legally precluded (by Proposition 209) from considering race in admissions. Throughout the past 15 years—most recently in a brief submitted to the Supreme Court—university officials have denounced race neutrality and pointed to the substantial drop in freshman black and Hispanic students at the system's two flagship schools, Berkeley and UCLA.

Yet race-neutrality has produced stunning benefits for minorities in the UC system as a whole, as shown in a data set that economists obtained from UC administrators. Black, American-Indian and Hispanic students made up 26% of all U.C. freshmen in 2010, up from 16% in 1997; the number of B.A.s earned by black and Hispanic students in four years rose 55% between 1995-97 and 2001-03, while the number with GPAs above 3.5 rose 63%.

Some details of Sander's research can be found at Volokh:

• After the first year of law school, 51 percent of black students have grade-point averages that place them in the bottom tenth of their classes, compared with 5 percent of white students. "Evidence suggests that when you're doing that badly, you're learning less than if you were in the middle of a class" at a less-prestigious law school, Mr. Sander says.
  
• Among students who entered law school in 1991, about 80 percent of white students graduated and passed the bar on their first attempt, compared with just 45 percent of black students. In a race-blind admissions system, the number of black graduates passing the bar the first time would jump to 74 percent, he says, based on his statistical analysis of how higher grades in less competitive schools would result in higher bar scores. Black students are nearly six times as likely as whites not to pass state bar exams after multiple attempts.
  
• Ending affirmative action would increase the number of new black lawyers by 8.8 percent because students would attend law schools where they would struggle less and learn more, and earn higher grades.
  
• With the exception of the most-elite law schools, good grades matter more to employers than the law school's prestige.