The CSI Effect
On TV, it's all slam-dunk evidence and quick convictions. Now juries expect the same thing--and that's a big problem
Even accredited crime labs, however, can make mistakes. Most publicly accredited labs gauge their proficiency through declarative tests, where lab workers know they're being tested. Although most labs do well on such tests, some experts question their ability to judge labs' day-to-day performance. And even in declarative tests, deficiencies can be glaring. According to 2004 proficiency results from one private testing service reviewed by U.S. News, a few labs failed to properly match samples on simple DNA tests, mysteriously came to the right result after making the wrong interpretation of the data, or accidentally transposed the information from one sample onto another. In a ballistics test, one lab matched a slug with the wrong test gun.
Such errors can have real-world consequences in court. In 1999, a Philadelphia crime lab accidentally switched the reference samples of a rape suspect and the alleged victim, then issued a report pointing to the defendant's guilt. Last year, a false fingerprint match led the FBI to wrongfully accuse an Oregon lawyer--and converted Muslim--of complicity in the al Qaeda-linked Madrid train bombings. The FBI later blamed the foul-up on the poor quality of the fingerprint image. "There are a number of cases that deal with what on the surface ought to be dead-bang evidence," says Fisher. "But it turns out it was the wrong result. Improper testing or improper interpretation of data left the innocent convicted."
For all the setbacks and scandals, science has made considerable progress in the courts since the advent of forensic investigation. In the 1600s, the evidence against two London "witches" accused of causing children to vomit bent pins and a twopenny nail was . . . a bunch of bent pins and a twopenny nail. So it must have seemed fairly revolutionary in the 1800s when a Brussels chemist named Jean Servais Stas devised a way to separate a vegetable poison from the stomach of a countess's brother to prove how he had been killed. Or when an English investigator around the same time solved the case of a murdered maid by matching a corduroy patch left in the mud at the crime scene to the pants of a laborer working some nearby fields.
"Obvious" problems. That doesn't mean forensics can always be believed, however, even when the data are accurate. As Sherlock Holmes said, "There is nothing more deceptive than an obvious fact." DNA is a case in point. While DNA testing is the most accurate of the forensic sciences, experts can make vastly different interpretations of the same DNA sample. Criminal justice experts say most lawyers and judges don't know enough about any of the forensic sciences to make an honest judgment of the veracity of what they are told. Prosecutor Mike Parrish in Tarrant County, Texas, decided to get a second opinion on his DNA evidence in a capital murder case three years ago after the local police lab amended its result to more strongly link his suspect to the crime. Suspicious, Parrish had the sample reanalyzed by the county medical examiner, whose results were much less definitive. In the end, Parrish said, because of the conflicting DNA reports, he chose not to seek the death penalty.