Forensic evidence is foolproof, right? It's how those clever cops on CSI
always catch the killer. DNA evidence springs innocent men from prison. Fingerprints nab the bad guys.
If only forensics were that reliable. Instead, to judge by the most comprehensive study on the reliability of forensic evidence to date, the error rate is more than 10% in five categories of analysis, including fiber, paint and body fluids. (Meaning: When the expert says specimen X matches source Y, there's a 10% probability he's wrong.) DNA and fingerprints are more reliable but still not foolproof.
Though a 2005 study in the Journal of Criminal Law & Criminology
suggests a fingerprint false-positive rate a bit below 1%, a widely read 2006 experiment shows an alarming 4% false-positive rate.
Yet the public sees errors as gross anomalies. Like the time the FBI wrongly linked an Oregon attorney named Brandon Mayfield to the 2004 Madrid commuter train bombing that killed 200 people. The FBI had claimed a 100% match between fingerprints found at the scene and Mayfield, who was held for two weeks in federal custody. When the Spanish National Police got the real perpetrator, an Algerian named Ouhnane Daoud, the FBI had to admit its mistake. Mayfield accepted a $2 million settlement from the government.
How can we preserve the usefulness of forensic evidence while protecting the public when it breaks down? The core problem with the forensic system is monopoly. Once evidence goes to one lab, it is rarely examined by any other. That needs to change. Each jurisdiction should include several competing labs. Occasionally the same DNA evidence, for instance, could be sent to three different labs for analysis.
Other reforms should include making labs independent of law enforcement and a requirement for blind testing. When crime labs are part of the police department, some forensic experts make mistakes out of an unconscious desire to help their "clients," the police and prosecution. Independence and blind testing prevent that. Creating the right to a forensic expert for the defense would help restore the imbalance in scientific firepower that too often exists between prosecution and defense. Private labs are subject to civil liability claims and administrative fines, giving them financial incentives to get it right.