On the opposite side of the ledger, DNA evidence has been used to convict the guilty. It's the latter use of DNA information that figures in a case to be argued before the Supreme Court this month.
Supported by virtually every other state, the Obama administration and local officials, including the district attorney of Los Angeles County, the state of Maryland is seeking court approval for taking DNA samples from individuals arrested for serious crimes and checking that information against a database that includes evidence from unsolved crimes.
The court should decline the request and affirm a ruling by Maryland's highest court that taking DNA samples from persons who haven't been convicted of a crime violates the Fourth Amendment's ban on illegal searches and seizures.
Several arguments for acquiring such information are being offered: that, like taking fingerprints, collecting DNA helps police to make sure a suspect is who he says he is; that, by revealing whether a suspect committed a violent crime in the past, it can shape decisions about whether the government should oppose bail; and that it is minimally intrusive, requiring only that a suspect swab the inside of his cheek.
But the most emotionally compelling argument for acquiring DNA at the time of an arrest is that it might implicate the suspect in another crime and help bring him to justice.
The problem is that the Fourth Amendment has traditionally required probable cause or at least reasonable suspicion before a search is conducted; the fact that someone has been arrested on suspicion of one crime doesn't create probable cause that he committed another. Unless he is convicted, he should have the same protection against unreasonable searches enjoyed by individuals who haven't been arrested (even though taking DNA samples from some of them might also clear up unsolved crimes).
That may not be a popular conclusion, but it's the correct one.