FANS of television shows such as “Law and Order” are familiar with the so-called “exclusionary rule”: when police obtain evidence of a crime through illegal means, the evidence is usually inadmissible in court. This rule, an outgrowth of the Fourth Amendment bar on “unreasonable searches and seizures”, deters police from violating citizens’ constitutional rights when undertaking criminal investigations. But the rule just became something closer to a suggestion: on June 20th the Supreme Court divided along gender lines in a 5-3 decision that introduces a loophole in rules for obtaining evidence that were developed more than 50 years ago.

The case, Utah v Strieff, involves a dodgy drug bust. Responding to an anonymous tip that drugs were being sold from a house in South Salt Lake City, Utah, detective Doug Fackrell started keeping an eye on the property. He didn’t see much from his unmarked car, but he did notice—in the several hours he spent watching over the course of a week—people visiting the home and then quickly leaving.

Without keeping track of how long one such visitor spent at the house, Mr Fackrell decided to stop the man, Edward Strieff, for questioning. The detective discovered, after a colleague ran his name through a database, that Mr Strieff had an open warrant for a traffic violation. Mr Strieff was arrested on the traffic warrant and searched. Mr Fackrell discovered a baggie of methamphetamine and drug paraphernalia in his pockets. He was then charged with drug possession. Mr Strieff challenged the charges by denying that the evidence against him was obtained lawfully.

The five men on the Supreme Court ruled against Mr Strieff. In an opinion written by Justice Clarence Thomas, the court quoted its own ruling in a case in 2006 that, “suppression of evidence...has always been our last resort, not our first impulse.” One circumstance in which the exclusionary rule does not apply is when “the connection between unconstitutional police conduct and the evidence...has been interrupted by some intervening circumstance.” Since it was the traffic fine (not the illegal questioning) that gave rise to the arrest, and since Mr Fackrell was “at most negligent” in questioning the suspect, his “errors in judgment hardly rise to a purposeful or flagrant violation of Mr Strieff’s Fourth Amendment rights,” the court held. The drugs can be used as evidence. Two biting dissents from Justices Elena Kagan and Sonia Sotomayor took sharp issue with this holding. “Do not be soothed by the opinion’s technical language”, Justice Sotomayor warned. “This case allows the police to stop you on the street, demand your identification, and check it for outstanding traffic warrants.”

The exclusionary rule has long been contested. Introduced in 1914 for federal prosecutions, it was not applied to all courts until the Supreme Court ruled on Mapp v Ohio 1961. Justice Tom Clark wrote in that case that police would have no incentive to stick to the Fourth Amendment when searching suspects if all the evidence they collect is admissible no matter how they come by it. Benjamin Cardozo, a Supreme Court justice in the 1930s, opposed it, asking why “the criminal is to go free because the constable has blundered”? Now begins the wait to see whether police forces change their behaviour to take advantage of the new powers the Court has just handed them.