This editorial will appear in tomorrow's print edition.

Taken too far, efforts to fix past racial discrimination tip over into reverse discrimination. But how far is too far?

The U.S. Supreme Court took a well-aimed stab at that knotty question this week when it ruled that a group of white and Hispanic firefighters in New Haven, Conn., shouldn’t have had their high test scores thrown out because the exam hadn’t produced any black candidates for lieutenant or captain.

A case like this turns on the details. It’s easy to imagine promotion requirements that are clearly unfair to minority candidates. In fact, American fire departments used to routinely screen out minorities – especially blacks – with a variety of practices unrelated to job performance. Passing out jobs to the sons and nephews of other firefighters, for example. Or simply not hiring and promoting blacks – or ostracizing any who did wind up getting hired.

New Haven, like a lot of cities, once had a nearly lily-white fire department. That’s why it went to considerable lengths to come up with a neutral promotion exam that tested for actual job skills.

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The history of that exam is at the heart of this case. Under the city charter, promotion must be done on the basis of merit as demonstrated in job-related tests. The city’s contract with the firefighters union further requires that the test be written and oral, as opposed to performance in simulated emergencies – a standard used by many other cities.

The test New Haven wound up using in 2003 was developed by a well-established Illinois company that specializes in police and fire exams. The firm relied heavily on minority firefighters as it developed the questions. Then it had the exam reviewed by 30 independent fire chiefs from similarly sized cities. Two thirds of those chiefs were minorities.

Despite that vetting, the test on this occasion proved an obstacle to all the black firefighters who took it that year. They threatened to sue, and the city decided to throw out the results. The successful white candidates then filed a lawsuit of their own – which carried the day Monday with the high court’s five-member conservative majority.

Given the facts of the case, the decision seems justified.

Maybe the city could have looked outside the department and encouraged more blacks to apply. But unions prefer to keep promotions in the family. The city might have relied on simulations, but the contract apparently forbade that. In any case, there’s no guarantee that the results would have been different with this pool of candidates. The skewed results might simply reflect the knowledge of these particular individuals.

If the same test repeatedly blocked any promotion of blacks, it should certainly be thrown out. But there’s no real evidence that it was inherently discriminatory or that it focused on anything but firefighting skills.

The city could have chosen not to use the exam. It can choose to look for something better next time. But once the exam has been given, the high-scorers shouldn’t be disqualified simply because too many of them are white – and chiefly because the city fears litigation. That looks too much like the panic-driven racial quotas that helped give affirmative action a bad name.