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LAST month, Steven Barnes was of the rape and murder of a 16-year-old New York schoolgirl in 1985. Barnes had been convicted of these crimes on the basis of forensic evidence, including testimony that soil on his truck tyres matched that at the crime scene. An imprint on the outside of his truck also supposedly matched the pattern of the jeans the victim was wearing when she was killed. But this year, tests showed that DNA samples from the murdered girl鈥檚 body and clothing did not match Barnes鈥檚, and he was freed after spending 20 years in jail.

The US National Academy of Sciences fears that miscarriages of justice like Barnes鈥檚 original conviction are all too common. In a highly critical report on the state of forensic science in the US, published last week, it questions the reliability of using techniques like hair or fingerprint analysis to link a person to a crime.

The report was welcomed by delegates at a meeting of the American Academy of Forensic Sciences in Denver, Colorado, last week. 鈥淚 think it鈥檚 long overdue,鈥� says Michael Baden, chief forensic pathologist for the New York state police. 鈥淚t brings criticisms of why so many innocent people get convicted in this country based on junk science.鈥� It is also the first independent review to lay out guidelines on how forensic evidence should be collected and analysed, and is likely to have global ramifications, says lawyer and forensic scientist Judith Fordham of Murdoch University in Perth, Western Australia.

鈥淭he report helps show why so many innocent people get convicted in the US based on junk science鈥�

But while forensic scientists agree that their discipline is due an overhaul, and indeed several are trying to make traditional techniques more robust (see 鈥淏loodstains鈥�), some say the report鈥檚 recommendations do not go far enough. 鈥淎 lot of them are naive,鈥� says Peter De Forest of John Jay College of Criminal Justice at the City University of New York.

At the heart of the problem is the absence of scientists at the front line of investigations, De Forest says, yet the report fails to address this. It is scientists who should determine which tests are most appropriate for a particular crime scene, he says. At present it is left to police officers and prosecutors to decide.

Also at fault is the system that allows prosecutors to bring expert witnesses into court to interpret evidence that they have had no hand in collecting. 鈥淚n some of the wrongful conviction cases it wasn鈥檛 the scientist who got it wrong, it was the prosecution that misinterpreted it,鈥� says De Forest.

Other forensic scientists are worried that the report鈥檚 heavy criticism of the way traditional techniques are used will lead to them being wrongly dismissed in favour of DNA evidence. Many traditional methods 鈥渁re getting short shrift鈥�, says Brian Gestring of Cedar Crest College in Allentown, Pennsylvania, who points out that such techniques can still play an important role in investigations 鈥� provided they are interpreted properly and not relied on uncritically.

鈥淚f I went to a crime scene and collected a blood sample from under a bleeding victim, I don鈥檛 know that I鈥檇 need $100,000 of DNA testing to tell me it鈥檚 the victim鈥檚 blood,鈥� Gestring says. 鈥淲hat I would need to do is look at the pattern of the blood to see if it was consistent with it being the victim鈥檚 blood.鈥�

While Gestring accepts that DNA can be extremely helpful in a minority of cases, 鈥渋t is not a panacea鈥�, he says. The belief in DNA鈥檚 infallibility sometimes means that DNA testing 鈥渉appens at the expense of all the other evidence, when more often than not it can say more than the DNA鈥�.

In fact, DNA is only helpful in around 10 per cent of murder cases, says Baden, and even in these cases there is usually plenty of other evidence as well. Where it can be extremely useful is in rape and sexual assaults, where a large amount of biological evidence has been left behind. Yet even DNA evidence may not be unequivocal. It too is open to interpretation, and there have been cases where it has gone wrong.

Both the report and its critics agree on one point: the reliability of many forensic techniques urgently needs to be quantified. 鈥淎 lot of the techniques used today are valid with certain caveats, but we need to go back and lay that sound foundation so these data are more reliable,鈥� says Bruce Goldberger, a forensic toxicologist at the University of Florida, Gainsville.

鈥淭he reliability of many long-standing forensic techniques urgently needs to be quantified鈥�

Achieving this will not be easy. How, for example, do you determine error rates for subjective judgements about how a strand of hair looks? This has led many analysts to argue that there is no error rate for techniques like hair or fingerprint analysis, which claim to be able to find unequivocal matches between samples. 鈥淚 don鈥檛 subscribe to that,鈥� says Gestring. 鈥淓ven computers have error rates. Let鈥檚 find out what they are.鈥�

In many areas of forensic evidence, scientists are beginning to do just that. Yet the tests are not the only source of error, says John Lentini at Applied Technical Services in Marietta, Georgia. He wants forensic testing to take a lead from drug testing, where those running a trial are 鈥渂linded鈥� to details about the drug or patient. 鈥淲e need to control expectation bias,鈥� Lentini says.

Despite the criticisms levelled against it, the NAS report has been widely welcomed as a reminder to those working in fields like fingerprints or ballistics that they need to do better. 鈥淭hey鈥檝e been very slow to come to the party,鈥� says Fordham. 鈥淣ow they鈥檙e going to have to.鈥�