For all the new attention focused on the tired old arguments about whether Clarence Thomas did or did not talk dirty to Anita Hill almost 25 years ago, his recently published memoir raises far more consequential issues. At the top of the list is Justice Thomas’s impassioned account in My Grandfather’s Son and recent media interviews of his conviction that racial affirmative-action preferences do African-Americans more harm than good.

The Thomas memoir brings this complaint to the fore at a time when a little-noticed battle is raging over an impressive and growing body of social-science research that offers some empirical support for Thomas’s view. This research suggests that preferences set many black students up for failure, as well as stigmatize the accomplishments of those who succeed and divert attention from the real causes of racial inequality.

So unsettling is this research — led by Richard Sander, a UCLA law professor and statistician — that affirmative-action champions are desperately seeking to deny Sander and other scholars access to the empirical databases that could provide even more graphic evidence of the costs of using heavy-handed preferences to engineer diversity.

Sander’s sophisticated statistical analyses suggest that racial preferences place so many black students into highly competitive law schools for which they are underqualified — and thus likely to fail — that there are actually fewer black lawyers than there would be if admissions were color-blind. Other scholars of diverse ideological views have joined Sander in seeking to shed more light on the validity (or invalidity) of his findings.

"Data from across the country," Sander (a moderate) and professor Vikram Amar (a liberal) wrote in a recent Los Angeles Times op-ed, "suggest to some researchers that when law students attend schools where their credentials (including LSAT scores and college grades) are much lower than the median at the school, they actually learn less, are less likely to graduate, and are nearly twice as likely to fail the bar exam than they would have been had they gone to less elite schools. This is known as the ‘mismatch effect.’ "

You might think that affirmative-action supporters such as the Society of American Law Teachers and the leaders of the California State Bar would be eager to learn whether preferences are, in fact, backfiring on intended beneficiaries. But so far they seem eager to avoid finding out. This at a time when virtually every selective law (and undergraduate) school systematically uses large racial preferences in admissions and when the American Bar Association’s accrediting arm is pushing hard for such preferences.

Indeed, some of the same critics who fault Sander’s studies because of limitations in the data at his disposal are seeking to prevent him and other researchers from accessing the far more specific and recent — but secret — data in the hands of the California State Bar. Its records of bar exam performance contain the nation’s best collection of information about disparities in the pass-fail rates and the scores of various racial and other groups.

Meanwhile, a lopsided majority of the U.S. Civil Rights Commission voted (over Democratic dissents) on August 28 to urge further research into the impact of racial admissions preferences on disparities in law school academic performance. In a widely ignored, 220-page report, the majority also called for legislation requiring federally funded law schools to disclose detailed data on the role of race in admissions decisions.

Like the law school and bar officials who seek to conceal evidence bearing on how well or badly preferences are working, Justice Thomas’s condescending critics treat him with contempt while exuding eagerness to prevent discussion of his views on the merits. Their major substance-avoidance gambit is to dismiss Thomas as a hypocrite who rode racial preferences into Yale Law School and would now deny the same benefits to other black students.

This is wrongheaded on three counts.

First, as a child of abject poverty who by dint of intelligence and hard work graduated ninth in a class of more than 500 from Holy Cross, Thomas was qualified for any law school in the country quite apart from his race.

Second, even if Yale Law did count Thomas’s race as a plus factor, it is no more hypocritical for him to decide that racial preferences are a bad idea than for Edward Kennedy to question the fairness of legacy preferences like the one that helped him get into Harvard College (before his expulsion and subsequent readmission).

Why, Thomas asked Jan Crawford Greenburg of ABC News, can’t people say: "Well, here’s somebody who went through it, and he has some problems with it based on his experience, and his intentions are as good as the people who are the authors of the initial policy?’ " The answer, as detailed in Greenburg’s excellent, in-depth article on the ABC News website, is that some people would rather sneer than think.

Third, the important question is not whether Thomas has moral standing to say that racial preferences harm intended beneficiaries but whether his view is supported by evidence. The answer, persuasively provided by Sander based on the limited data so far available to him, is a qualified yes.

Of course, racial preferences clearly have benefits as well as costs, and many African-Americans (and other minorities) are very glad to have enjoyed them. Thomas unfortunately ignores such complicating factors. But Sander’s evidence confirms Thomas’s view that for many other supposed beneficiaries, preferences have turned out to be a curse.

While a light finger of racial preference on the scales may do some good and little harm, the preferences in elite law school and college admissions are not light. And the resulting gap in average academic qualifications that separates white (and Asian) students from black (and to a lesser degree Hispanic) students at almost all selective universities is so large that many black students struggle to keep up and have very low average grades and high dropout rates.

This, in turn, aggravates what Thomas calls "the stigmatizing effects of racial preference." When he was at Yale, his book recalls, he feared that the admitted-because-black sign that had effectively been hung around the neck of every black student "would be used forever after to discount my achievements."

As detailed in my December 4, 2004, column, Sander has documented stunning racial disparities in academic performance among law students, whose success and failure rates on bar exams dramatically illustrate the downside of affirmative action. (There is reason to suspect similar effects in undergraduate school, but with no exit exam to measure them.) In testimony before the Civil Rights Commission last year, Sander reported that:

• "At the vast majority of law schools, half of all blacks are ending up in the bottom tenth of their class.

• "Blacks are two and one half times more likely than whites to not graduate from law school.

• "Blacks are more than four times as likely to not pass the bar on their first attempt.

• "Blacks are more than six times as likely as whites to never pass the bar after multiple attempts."

Things are getting worse, Sander added: "As best we can tell, today only about one-third of blacks starting law school will graduate, take the bar, and pass on their first attempt," compared with more than two-thirds of whites and about half of Hispanics.

The main reason for these shocking disparities, according to Sander, is the use of such large racial preferences at all selective law schools as to place "nearly all blacks at an enormous academic disadvantage in the schools they attend." This is because the most-selective (first-tier) schools admit blacks who are academically qualified only for second-tier schools; second-tier schools thus fill their de facto racial quotas by admitting blacks who are qualified only for third-tier schools; and so on.

The result, Sander says: "Students at the bottom of the class apparently learn less than the same student would learn at a lower-tier school where the student was closer to the middle of the class," and thus are less likely to pass the bar exam.

This in turn means that the racial-preference system produces fewer black lawyers than would a color-blind system, the data available to Sander so far suggest. He has also documented how racial hiring preferences at the nation’s leading law firms lead to "disparities in expectations and performance that ultimately hurt the intended beneficiaries." (See my June 17, 2006, column.)

Sander admits that definitive conclusions are difficult because he has not had access to the more specific data that racial-preference champions are trying to deny him. The apparent thinking behind this secretiveness is, as the Sander-Amar op-ed contends, that "no one should seriously attempt to get to the bottom of racial disparities in bar performance because the attempt itself would make more people aware of the disparities!"