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Bullet lead analysis, which was the subject of a joint Washington Post and 60 Minutes report on Nov. 25, makes a useful case study in the rise and fall of a forensic technique because it has a definite beginning and ending. It began during the investigation of the Kennedy assassination. And it ended on Sept. 1, 2005, when the FBI Laboratory, the only laboratory in the United States to use the technique, announced that it was discontinuing it. Of course, while the technique has ended, the story is not over, as the recent report demonstrates. Hundreds of individuals remain in prison based in part on bullet lead evidence, and some of them may be innocent. The FBI’s failure to follow up on these cases after discontinuing the technique was the focus of the Nov. 25 report. Although bullet lead analysis was a fairly rare technique, its story reveals important lessons about forensic science. First, the simple chronology is revealing. During the 1960s, the FBI explored using chemical analytic techniques to measure the amounts of trace-element impurities in bullets recovered from crime scenes that were too mangled for conventional striation analysis. The techniques were standard and accepted analytic techniques. But the inference to make from the analysis was trickier. What did it mean that the chemical composition of a crime scene bullet was similar to that of bullet taken from a suspect’s cache of bullets? Well, that obviously depends on how variable bullet chemical compositions actually are. The FBI assumed that no two cauldrons of bullet lead would have the same chemical composition. It also assumed that each cauldron of lead would have the same chemical composition throughout. Neither of these are actually true. But no one knew that until at least 1991, more than 20 years after FBI experts began testifying in court about chemical “matches” between the defendant’s bullets and the crime scene bullets, when an FBI study suggested that these twin assumptions might not be correct. Even that didn’t stop the use of the technique. Only in 2000 did independent scientists publish data testing these two assumptions. They found that different “melts” of lead could be chemically indistinguishable and that they were not necessarily uniform throughout. It doesn’t quite seem like the way the relationship between forensic science and law was supposed to work. Wouldn’t one expect that you test your assumptions before you start testifying in court rather than after? But this brings us to our second point. It’s all about the testimony. Testimony in court is the goal of all of forensic analysis, whether it actually occurs or its threat is merely a chip in the bargaining process. That, after all, is why they call it “forensic” science. It is science that “speaks” in court. And it was in the testimony that the bullet lead analysis got really out of whack. Even if one accepted the FBI’s assumptions, a chemical match only meant that the bullets came from the same batch of lead, which could contain many millions of bullets. But, as documented in both the National Research Council report on the technique and in the Post/60 Minutes report, FBI expert witnesses sometimes testified that a chemical match between the crime scene bullet and the suspect’s bullet meant they came from the same box of bullets. No matter what one thinks about the value of bullet lead analysis, such testimony exaggerates its value. Goal should be valid testimony In the 60 Minutes interview, retired FBI Laboratory Director Dwight Adams defended “the science” while admitting that the testimony given in some cases was scientifically unsupportable. This artificial separation between “the science” and the inferences drawn from it is common in forensics. Forensic scientists and reformers alike have spent more time thinking about whether “the science” is valid than about whether the testimony is valid. But, as the bullet lead saga demonstrates, even valid science is useless if it can’t produce valid testimony. Although bullet lead illustrates both these problems, neither of them is limited to that technique. Many other forensic techniques also followed the “testify-first-validate-later” approach. And courts allowed the testimony without questioning it. Fingerprint identification, for example, still remains to be validated. Many of these techniques have not developed scientifically supportable ways of telling jurors what a “match” means. Bullet lead witnesses told juries one of two things. Sometimes they told the juries something scientifically meaningless but rhetorically effective: that the bullets “matched.” Other times, they told the jury something scientifically unsupportable: that the bullets came from, or probably came from, the same box. Many other trace-evidence techniques, such as fingerprinting, bite marks, and firearms and tool marks, use these two forms of testimony. It turns out the science of bullet lead analysis couldn’t tell us much useful about whether a suspect was guilty. But the testimony could get them convicted. It’s time to start thinking more about what forensic experts actually say to jurors. It’s time to start putting the forensics back into forensic science. Simon A. Cole is an associate professor of criminology, law and society at the University of California, Irvine. This piece is adapted from his article “Where the Rubber Meets the Road: Thinking about Expert Evidence as Expert Testimony,” forthcoming in the Villanova Law Review.

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