Neil Barrett

Growing up - or at least, growing older - brings with it a plethora of worries and concerns, alongside the thinning hair and the expanding waistline. You gain experience of the world, and its myriad aspects of unfairness and difficult decisions. In my life, this is most obvious in the work I do as a computer expert witness.

Through computer evidence - the records of email, the relics of Google searches, the downloaded pictures - you can achieve a tremendous insight into a defendant's morals and motives, their needs and desires. I have worked on murder cases in which the defendant has downloaded endless amounts of the foulest sadomasochist material imaginable; hacking cases redolent with teenage angst and frustration; and paedophile cases that make me want to weep or worse, and to never, ever let my children out of my sight, let alone on the internet.

High on my personal list of worries are the cases where I am positive - certain right down to the bone - that the defendant is guilty, but that the evidence has been tainted or spoilt.

Whether by careless handling, viral intrusion, or deliberate contamination, the evidence is muddied so severely that, with the best will in the world, I don't believe that the jury can be expected to understand it. Cases like these are often dropped, and the defendant walks free: to hack again, to rob again, to harass again, or worse - to abuse children again.

These problems alone are bad enough, and leave a foul taste in the mouths of the police, the prosecution and, I suspect, even the defence teams. But much worse are those court cases in which the opposing computer expert witnesses have allowed themselves to be dragged into a spoiling game, to throw any arguments, even the most spurious, against the evidence in hand.

The role of the expert witness is simple, at least in principle: it is to help the court to understand the nature and meaning of technically complex evidence that they cannot be expected to understand without assistance. Computer evidence often requires expert explanation, because it often concerns topics such as date stamps, file contents and disk geometry.

It should, though, be a simple task: explain how computers work, describe the evidence in layman's terms, and stand ready to explain further from the witness box.

There are, though, a small number of expert witnesses who raise the most arcane and unlikely explanations for why material might be found on a personal computer. In paedophile cases, for example, I now constantly have to face the question of pop-ups, redirectors and Java - even in cases where it is clear that defendants have created their own archives of unlawful pictures and have lovingly browsed them over a period of many weeks.

The defendant has to be presumed innocent until found guilty; but where all the evidence points unequivocally at their guilt, let's try to avoid abstruse arguments, which could exonerate them by confusing the jury. Please.

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