Knock knock.

Never mind “Who’s there?” In fact, leave off the knock, too. Last month the U.S. Supreme Court said that cops with a warrant need not knock, let alone identify themselves as the police.

More specifically, the court said the “exclusionary rule,” under which evidence from an illegal search is excluded at trial, no longer applies in such a case. The decision was a close call – the justices split 5-4.

The facts before the court were clear. The police, warrant in hand, burst into the defendant’s house without announcing themselves. Taken by surprise, the defendant was caught with drugs.

The way things used to work, the evidence should have been excluded as the fruit of a tainted search. Traditionally, the “exclusionary rule” balanced the rights of the accused and the interests of effective law enforcement by tossing evidence resulting from police misconduct.

The idea that no one, not even the police, should burst into your home without announcing themselves is a longstanding American tradition. The Supreme Court stuck with this tradition in principle.

Where the court went off course was in not understanding the value of the exclusionary rule as a sensible mechanism to assure proper police conduct.

Technically, the court did not disturb the balance between law-enforcement effectiveness and citizen rights, leaving intact the requirement that police announce themselves. The new ruling formally changes only the enforcement mechanism.

You can still file a complaint or sue the police if they burst into your house unannounced. But evidence they find is now admissible at trial.

It’s a bad decision. The old exclusionary rule kept the police inside the bounds set by society.

As a practical matter, the Supreme Court KO’d the no-knock rule by eliminating the enforcement mechanism. And dumping the exclusionary rule without replacing it with something equally strong is a mistake.

On the surface, the exclusionary rule seems like a good target for elimination. It hands the guilty a get-out-of-jail-free card when law enforcement breaks the rules, but it doesn’t do anything at all to protect law-abiding citizens against police misconduct.

In a perfect world it’s the law-abiding, not the law-breakers, who would get the protection. Nonetheless, practical arguments about incentives favor keeping the traditional exclusionary rule.

The first argument is political. We simply don’t have an alternative mechanism for making sure that the police follow the rules. Under the American constitutional system the courts can enforce rules by excluding tainted evidence, but only the legislative branch can make it easy to sue the police for large damages. The right way to get rid of the exclusionary rule would be for Congress – or our state Legislature – to put an alternative, effective enforcement mechanism in place first, and only then relax the exclusionary rule.

But there’s a more subtle argument favoring the exclusionary rule over a sue-for-damages rule. As important as it is that the police respect everyone’s rights, it’s also critical that they go after criminals aggressively.

If we’re asking our police to be aggressive, we’d better think what it would be like to walk in their shoes, and ask what happens when an officer is accused, fairly or not, of stepping over the line.

If an officer can be sued easily, he or she is going to think twice before taking an aggressive action, because the risks of being sued are borne personally. So a sue-the-cops solution overly discourages aggressive policing.

In contrast, under the exclusionary rule the incentive to stay within the rules is that you blow your case if you break them … but the officer isn’t at personal risk.

Our system vacated by the Supreme Court used the exclusionary rule to control everyday behavior and the right to sue (but not to sue easily) to prevent really egregious violations. It’s been a good balance.

The exclusionary rule has largely succeeded in limiting police misconduct. It provided incentives for officers to follow the rules without putting them in fear of punishment for minor mistakes. The exclusionary rule has served us pretty well. The court erred in not leaving well enough alone.

Dick Startz is Castor Professor of Economics at the University of Washington. He can be reached at econcol@u.washington.edu.