The Justice Department and the Federal Bureau of Investigation have been making headlines for what have been termed "mistakes" in forensic testimony involving hair comparison. There is no doubt that the testimony offered in countless criminal trials was wrong. What is in doubt is whether a term as benign as "mistake" is appropriate in these cases. The FBI Laboratory was not unwittingly using a faulty machine. According to a massive review of the testimony provided by that unit, more than 95 percent of trials included testimony that misstated evidence in ways that favored the prosecution.
Among the cases where evidence was misrepresented were 32 death sentence cases, including 14 where the convicted defendant has already been put to death. It is not clear that proper testimony, with the backing of science, would have changed the outcomes of these cases. The mere possibility of that, however, should be enough to inspire scrutiny into the Justice Department and FBI.
Federal criminal cases are often complex and tend to involve serious penalties. Being charged with a federal crime is a terrifying experience. The potential for harm in misrepresenting forensic evidence is substantial. The more complicated an issue, the less likely jury members are to feel comfortable substituting their own judgment. If a purported expert says a hair follicle is a match for an accused person, what tools does a layperson have to deny that claim? If the hair later turns out to have come from a lab worker's dog, who will answer for that mistake?
Source: Slate, "Pseudoscience in the Witness Box," by Dahlia Lithwick, 22 April 2015