Reason contributors Glen Whitman and Roger Koppl have an article in the journal Law, Probability & Risk on rational bias in forensic analysis. The outright frauds and charlatans in the forensics field are bad enough. But Koppl and Whitman focus on the structural biases inherent in the way forensic analysis is often performed, biases that likely afflict even well-intentioned analysts. From the abstract:
Specifically, forensic examiners' conclusions are affected not just by objective test results but also by two subjective factors: their prior beliefs about a suspect's likely guilt or innocence and the relative importance they attach to convicting the guilty rather than the innocent. The authorities—police and prosecutors—implicitly convey information to forensic examiners by their very decision to submit samples for testing. This information induces the examiners to update their prior beliefs in a manner that results in a greater tendency to provide testimony that incriminates the defendant. Forensic results are in a sense 'contaminated' by the prosecution and thus do not provide jurors with an independent source of information. Structural reforms to address such problems of rational bias include independence from law enforcement, blind proficiency testing and separation of test from interpretation.
Koppl and I co-wrote a piece for Slate a couple of years ago laying out some of those structural reforms.
The problem goes back to the fact that forensics isn't really science. With the notable exception of DNA testing, most fields in forensics were invented and developed by police agencies, not scientists. So they have evolved over time not to better seek out the truth, but to help law enforcement personnel prove a hunch or theory. Hence the general absence in most forensics of critical components to the scientific process like peer review and blind testing. The forensic evidence is then still presented to juries with all the gloss, polish, and impressive-sounding vernacular of hard science.
The other problem is that while science might be described as a perpetual journey toward truth, once a jury hands down its verdict, the criminal justice system tends to put a premium on finality and closure. So even though science may later show us that the forensic evidence used to convict someone 10 or 20 years ago is flawed, overstated, or simply false, the courts have been reluctant to reopen those cases.