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This 麻豆传媒 article, usually accessible only to subscribers, is made available for free by the Museum of Applied Arts and Sciences in Sydney, Australia

A WOMAN is found strangled and partially burned in her house. DNA matching her ex-partner 鈥� who claims he hasn鈥檛 seen her for several months 鈥� is lifted from her pyjamas. The man claims his DNA must have got there via their child鈥檚 clothing or toys. Would you believe him? Read on before reaching your decision.

The conviction last week of Gary Dobson and David Norris for the murder of Stephen Lawrence in the UK in 1993 has thrown the forensic analysis of so-called 鈥渢ouch鈥� evidence into the spotlight. The case hinged on tiny fragments of blood, hair and fibres found on Dobson鈥檚 clothes. The defence team argued that the fragments, which were fewer in number than would be needed to fill a teaspoon, could have got there by contamination. The forensic scientists who testified dismissed such contamination as highly unlikely.

Although there was additional evidence against Dobson and Norris that helped to secure their conviction, several new studies illustrate that contamination is an issue that should be given more consideration than suggested by the prosecution. The research shows the need for caution when interpreting touch evidence 鈥� particularly as it becomes used more widely.

鈥淧olice are increasingly trying to find these invisible stains, because so far as they鈥檙e concerned, DNA is a very cost-effective crime-solver,鈥� says Allan Jamieson of the in Glasgow, UK. 鈥淭he finding of someone鈥檚 DNA implies their presence. But presence is not a necessary consequence of finding someone鈥檚 DNA.鈥�

鈥淔inding a person鈥檚 DNA implies their presence, but presence is not a necessary consequence of that find鈥�

Mariya Goray of Victoria Police Forensic Service Centre in Australia and her colleagues re-enacted several scenarios loosely based on real events in which DNA from a defendant was found on a victim鈥檚 clothes or a murder weapon, and where the defence argued that it could have got there indirectly. Mimicking the scenario described in the intro, Goray asked a volunteer to handle a child鈥檚 vest and wooden toy for 1 minute before these objects were rubbed against the front of a lab coat, which represented pyjamas. They found that enough of the volunteer鈥檚 DNA transferred to clearly identify him (Legal Medicine, ).

In a separate study, Goray and colleagues illustrated how easily DNA can transfer within and between items from a crime scene during transport to the lab, in a study involving objects such as used cigarette butts and bloodied knives (). 鈥淭here is a distinct possibility for the misinterpretation of a result that could impact negatively on the criminal investigation,鈥� says Goray.

Problems may also arise when evidence or bodies are examined on supposedly clean laboratory surfaces. Thorsten Schwark at the University Hospital of Schleswig-Holstein in Kiel, Germany, and his colleagues swabbed the shoulders and buttocks of six cadavers after they had rested on autopsy tables. They found that four of them were contaminated with DNA from bodies that had previously rested on the table 鈥� in two cases with DNA from more than one person (Forensic Science International, ). 鈥淭his may seriously influence the interpretation of trace analysis results taken during autopsies,鈥� says Nicole von Wurmb-Schwark who was also involved in the work. For example, fragments of DNA from previous autopsies could mask or confuse DNA profiles from genuine assailants.

, former principal scientist at the UK鈥檚 Forensic Science Service, now at Oslo University in Norway, says scientists should not dismiss the possibility of contamination, particularly where tiny amounts of DNA are concerned.

鈥淒NA is ubiquitous,鈥� he says. 鈥淭here are lots of examples where inadvertent transfer of DNA has happened. The problem is these evidence items are often kept in plastic bags and if you have got heavily bloodstained items, for example, then DNA is going to transfer across items.鈥�

That鈥檚 not to say touch evidence shouldn鈥檛 be used to help solve crimes, but jurors need to be presented with information about its limitations or wrongful convictions may ensue, while other convictions may fail or be overturned on appeal.

鈥淚 think that when we鈥檙e dealing with very low levels of DNA we need to report that a DNA profile matches, but as to how and when it got there we just don鈥檛 know,鈥� says Gill.