Last week, the state of Mississippi terminated its 20-year relationship with medical examiner Dr. Steven Hayne. Hayne has come under fire from fellow medical examiners, criminal justice groups like the Innocence Project, and one of the authors of this article for his impossible workload, sloppy procedures, and questionable court testimony. In the early 1990s, Hayne and his frequent collaborator, now-disgraced forensic odontologist Dr. Michael West, helped secure murder convictions for Kennedy Brewer and Levon Brooks, both later proven innocent through DNA testing. The two were released from prison earlier this year.

Mississippi is hardly alone when it comes to bad forensic science. It now appears that Washington, D.C., may have to retry Angela O'Brien for the 2000 killing of her 2-year-old goddaughter, Brianna Blackmond, after revelations that the prosecution's star forensic witness, a physicist named Saami Shaibani, lied about his credentials in a Wisconsin murder case. These are only the most recent and dramatic examples of forensics fraud to make the headlines. Over the years, there have been plenty of other hucksters and charlatans happy to take advantage of the ignorance of juries, prosecutors, judges, and defense attorneys in very complicated and difficult-to-understand disciplines.

But the charlatans are only half the story. Courts have also missed plenty of mistakes from well-intentioned, conscientious scientists, too. In fact, these may be even more common—and harder to catch. Studies show that crime lab fiber, paint, and body fluid analyses, for example, may consistently have error rates of 10 percent or higher. The error rate in fingerprint analysis is possibly between 1 percent and 4 percent. And bite mark evidence is notoriously unreliable though still widely used. The Chicago Tribune reported in July that L. Thomas Johnson—one of forensic odontology's pioneers—has been attempting to use statistical models to shore up the reliability of this discredited field. But Johnson's efforts have been hampered by new DNA testing in a 1984 murder, which concluded that the man convicted of the crime was not the source of saliva found on the victim's sweater. Johnson testified for the prosecution in that case.

The use of forensic science in criminal trials is critically important. But reforms of the system are also desperately needed. It's not enough to weed out the incompetent scientists. We need to begin to monitor even the good ones. One major barrier to improving forensic evidence in criminal trials is that in most jurisdictions, the state has a monopoly on experts. Crime lab analysts and medical examiners (and to a lesser extent DNA technicians) typically work for the government and are generally seen as part of the prosecution's "team," much like the police and investigators. Yes, science is science, and it would be nice to believe that scientists will always get at the truth no matter whom they report to. But studies have consistently shown that even conscientious scientists can be affected by cognitive bias.

A scientist whose job performance is evaluated by a senior official in the district attorney or state attorney general's office may feel subtle pressure to return results that produce convictions. In cases in which district attorneys' offices contract work out to private labs, the labs may feel pressure—even if it's not explicit (though sometimes it is)—to produce favorable results in order to continue the relationship.

Cognitive bias can be even subtler. For some experts, merely knowing the details of a crime or discussing it with police or prosecutors beforehand can introduce significant bias to a lab technician's analysis.

A Mississippi judge has recently taken the unusual step of allocating several thousand dollars in county funds so a defendant, accused of murdering a Jackson State University co-ed last November, can hire a forensic expert to examine the evidence in the case, scheduled for trial in September.

With forensic evidence - fingerprints, DNA, ballistics, bloodstain patterns, footwear analysis and the like - a significant factor in more and more verdicts, let's hope Judge Swan Yerger's recent action sets a national precedent. Let us explain why.

In today's "CSI" world, forensic scientists, like the television character Gil Grissom, may have overtaken lawyers as the most influential players in courtroom dramas. The evidence they analyze and present as part of the prosecution team is often the deciding factor in whether a defendant is found guilty or innocent.

But research indicates that forensic evidence is often flawed. So, in fairness, defendants should have a right to forensic expertise, just as they have a right to an attorney.