11 August 2010

Time to rethink DNA evidence

THEY call it the CSI effect. The seemingly endless succession of glossy TV shows about forensic science has created a popular perception that the dazzling array of techniques now available can produce fast and firm evidence for convictions.

It is well known that traditional methods such as bite-mark and fingerprint analysis rely to some extent on analysts’ opinions, and as such they are viewed with a healthy measure of scepticism. However, DNA evidence has so far escaped this treatment.

Profiling is generally seen as routine, infallible and always able to get its man. One recent study found that jurors rated DNA evidence as 95 per cent accurate, while mock trials showed that they were more likely to convict when presented with DNA evidence than other types of evidence.

“Jurors see DNA evidence as highly accurate and are more likely to convict when it is presented”

But, as we demonstrate in this issue, DNA profiling is far from perfect, and in many cases can run into the same problems of subjectivity and bias seen with the more old-fashioned techniques.

This is an evolving problem, one rarely encountered in the early days, when analysts only tended to tackle large samples of DNA from one or two people. Forensic scientists are now increasingly swabbing crime scenes for DNA evidence as a matter of routine, which means samples are often very small or contain DNA from more than one person. These factors make analysis far more challenging.

Even though doubts were raised about DNA evidence as long ago as the early 1990s, albeit for quite different reasons, it is still seen as rock

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