THANKS to TV shows like CSI: Crime Scene Investigation, forensics is perceived as a glamorous science. In reality, of course, it is anything but, and in some areas it isn鈥檛 particularly scientific either. Fingerprints, tool marks and similar evidence can be invaluable in bringing a criminal to justice. The problem is that expert witnesses have tended to exaggerate what forensic techniques can tell us.
Forensic science has to transform itself. For a role model it could look to DNA analysis, with its careful attention to what the evidence can and cannot tell us. There are signs a change is already under way, but it is not happening fast enough. The big obstacle is the courts. Expert witnesses command great authority in the eyes of jurors and judges, and problems arise when the so-called experts claim greater precision for their techniques than they can support.
In the US, some courts have adopted screening mechanisms to try to prevent untrustworthy experts from testifying, but this is a problem when judges without extensive scientific training have to decide whether a practitioner knows what they are talking about. Courts have traditionally deferred to 鈥渢he scientific community鈥� to help them distinguish respected practitioners from charlatans. However, in some forensic sciences, many of the practitioners are not scientists.
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This was the difficulty faced by the Supreme Judicial Court of Massachusetts in the recent Commonwealth vs Patterson murder trial. The point at issue was the reliability of fingerprint identification. The state argued that, since all fingerprint examiners believe fingerprint identification is fail-safe, the court should back it. The defendant argued that a broad cross-section of scientists and scholars who had studied the issue agreed that the reliability of fingerprinting will never be known until its accuracy is measured 鈥� something that has yet to be done.
The court backed the practitioners. It ruled that, in order to be admissible evidence, fingerprinting needed to be accepted only among practising fingerprint examiners. This was a victory for the state, and a defeat for science. According to the court鈥檚 logic, any community of practitioners is now entitled to assess its own reliability.
This is unfortunate because many forensic disciplines appear ill equipped or disinclined to take a rigorous empirical approach. A century of fingerprint practice went by before the first attempt to measure its accuracy was published earlier this year (Journal of Forensic Identification, vol 56, page 55). The FBI recently laid out an ambitious research agenda to place fingerprinting on a more scientific footing, but only after it was forced to admit that a false fingerprint identification had wrongly implicated an Oregon lawyer in the Madrid train bombings of 2004.
鈥淢any forensic disciplines appear ill equipped or disinclined to take a rigorous empirical approach鈥�
A good illustration of the lack of research underpinning much forensic science is the use by the FBI of an esoteric technique called comparative bullet lead analysis (CBLA), which the FBI discontinued in 2004. When bullet fragments were too badly damaged to allow for the examination of striation marks, which can match a bullet to a particular gun, the FBI would instead analyse their metallurgical composition. If bullets from a crime scene contained similar proportions of trace elements to bullets found in a suspect鈥檚 possession, FBI scientists argued this was convincing evidence that they came from the same source.
CBLA rested on untested assumptions about the diversity of lead sources and their chemical consistency. The validity of these assumptions was only addressed in the 1990s. It turned out that lead sources are neither entirely heterogeneous nor internally homogeneous (快猫短视频, 20 April 2002, p 4). In 2004, the FBI discontinued CBLA after a critical report from the National Research Council (NRC).
What does this say about the state of forensic science? At a National Academy of Sciences (NAS) colloquium in Washington DC last November, Ronald Singer, former president of the American Academy of Forensic Sciences, said the CBLA episode showed forensic science was 鈥渟elf-correcting鈥�. That is a generous interpretation, considering that it took more than 30 years for the FBI to fund the NRC report.
Here鈥檚 another way of viewing it: the FBI鈥檚 use of CBLA turned the usual relationship between law and science on its head. Rather than performing scientific research to determine what can responsibly be said about a form of evidence, and then offering such testimony in court, it appears the FBI testified first and did the science later.
Change is coming. 快猫短视频s from many disciplines are taking an interest in forensic science, and the NAS is preparing to create a panel to study its needs. But unless the courts stop rewarding practitioners for ignoring science, the revolution is likely to take longer than it should.