Last week, Texas state senators heard testimony from top university officials about changes to a state law that automatically admits top-ranked high school students to the University of Texas. UT President Larry Faulkner told senators that the law is outdated because schools can now consider race instead.

Affirmative action, once abolished by court order in Texas, is legal again due to a more recent Supreme Court decision. But Texas lawmakers should not be fooled into thinking that preferences are good policy.

Colleges swear that preferences benefit minority students. Former university presidents William G. Bowen and Derek Bok famously argued in their book, The Shape of the River, that affirmative action offers impressive results at “a tolerable cost.”

But recent research has not been kind to their claims. Affirmative action lowers grades and graduation rates, reduces minority applicants to graduate schools and impoverishes relations between classmates.

Economists Audrey Light and Wayne Strayer analyzed data from thousands of students at public and private colleges and universities. They found that a student’s chance of graduating was significantly lower if there was a poor academic fit between student and college. Students do best and are most likely to graduate in schools with similarly prepared peers.

But selective schools interfere with the student-college matching process for minority students when they employ preferences. Partly as a result, minority students are three times more likely than others to drop out, and those who graduate finish, on average, in the bottom 25 percent of their college class.

Poor grades make students less likely to go to graduate school. Sociologist Stephen Cole found that preferred students were less likely than counterparts at less selective schools to enter doctoral programs later in life.

Sociologist Douglas Massey and his colleagues found that many minority students at selective schools perform worse than they otherwise might because of anxiety about confirming negative stereotypes.

Preferences also harm race relations between students. Mr. Massey writes, “Perceptions of distance from ‘affirmative action beneficiaries’ carry important implications for the general tone of race relations on campus.” Affirmative action generates distrust that works against the goal of diversity: respect and understanding between students from different backgrounds.

For high school seniors, early April may mean anxious trips to the mailbox to search for thick envelopes from selective colleges. Acceptances include glossy brochures promising four perfect years and a glittering future.

These are empty promises for some students. Selective schools accept black and Hispanic applicants they have reason to believe will not thrive. Determined to create a diverse freshman class, they allow minority students to pay the price.

The negative consequences of affirmative action don’t appear in college catalogues. Admission letters don’t warn students whose chances of graduating are low because of preferences. At the University of Texas at Austin, African-Americans admitted under preferences had a six-year graduation rate of 56 percent. But you cannot find this statistic on any school brochure.

Selective schools like UT-Austin will tolerate affirmative action’s costs because they want to achieve diverse campuses and would rather not admit fewer elite white and Asian applicants.

But preferences are uniquely harmful because they isolate minority students. Mr. Massey writes that they “produce a freshman class composed of two very distinct subpopulations. On one hand are whites and Asians, on the other are Latinos and blacks.” Universities that use class-rank policies or other alternatives avoid this kind of isolation.

But selective schools like their exclusive cachet, reflected by high average SAT scores and top rankings in U.S. News. So, if they are allowed to do so, they will attain diversity by mixing less qualified minority students into classes otherwise composed of uniformly overachieving whites and Asians. Then, they try to hide the size of academic disparities that are obvious to students and teachers.

Lawmakers and voters should not fall for affirmative action’s empty promise. Public institutions like UT should put students ahead of faculties and education ahead of prestige by maintaining race-blind admissions policies. Only in this way are minority students well served.