To the untrained eye, it’s unremarkable: the tiniest defect on a thumb nail. To Professor Sue Black, one of Britain’s leading forensic anthropologists, it is the piece of evidence that allowed her to identify a sex offender and helped convict Scotland’s largest known paedophile ring.

It is also confirmation that a science Black is pioneering may soon rank alongside finger-printing and DNA analysis as one of the key weapons in the fight against crime. The art of reading hands, for centuries the preserve of fortune tellers, may yet bring the most evasive of abusers to justice.

Black, a warm, bluff woman in her forties whose job has taken her from the mass graves of Kosovo to war-torn Iraq, is ambivalent about television series such as CSI and Silent Witness. They trivialise, sensationalise and generally misrepresent her profession. But the story that unfolds in her office at the University of Dundee is as gripping as any thriller.

The network of paedophiles had links stretching from Australia to Massachusetts. At its heart was a small group of Scottish men — husbands, church elders, charity workers, teachers and bank employees. They were well-educated, well-respected and good at covering their tracks.

Neil Strachan, a 41-year-old maintenance engineer and former secretary of the Celtic Boys Club in Edinburgh, was a ringleader of the paedophile network. He had committed his first offence at the age of 17 and been sentenced to three years in jail in 1997 for abusing children. Strachan, who is HIV-positive, was caught when he took a computer to be fixed. The repair technician found an abusive image of a child, one of 7,000 images later recovered by police.

The most infamous of these came to be known as “the Hogmanay image”, in which an unidentified man was shown engaging in graphic sexual abuse of a toddler. Identifying the man in the picture would be crucial to smashing the ring. There was, however, very little to go on.

“We were approached by Lothian and Borders police very late in the investigation,” says Black. “Our job was to compare images of the offender with images of the suspect. The defence had instructed the police to take some photographs of Strachan’s thigh. In those pictures, Strachan is holding a photographic scale, which, coincidentally, gave us two very good images of his thumbs. In the Hogmanay image, there was also an image of a man’s right thumb. You could say it was fortuitous.”

Strachan’s right thumbnail had an unusual morphology, a defect present from when he was in the womb. The identical defect was on the thumb in the Hogmanay image. Before Black could convince the jury, she had to demonstrate just how individual a hand is.

“We’re not at all symmetrical,” she says, laying her freckled arms on the table. “If you look at the pattern of veins on your right hand and compare it with the pattern of veins on your left hand, they are completely different. It’s the same for the blemishes on your skin and the creases on a knuckle. Within a hand, there is a tremendous amount of information.

“What we can’t tell yet is what the likelihood is of anybody else having the same pattern,” says Black. “This is very much pioneering science. We were careful to say to the court that we could not say definitively that it was him. But that is true in almost every case of identity. Every technique we use is based on probability, even DNA.”

Black’s methodical deconstruction of the images convinced the jury. Six members of the network were jailed for a total of 45 years in the high court last week. The two ringleaders, Strachan and James Rennie, will be sentenced next month. The technique has been used in another paedophile case. Dean Hardy, who had travelled to Thailand to abuse children, confessed when presented with Black’s findings.

“I don’t know that anybody else is doing this, which surprises the hell out of me,” says Black. “The great Renaissance painters all knew that your entire life is visible in your hands.”

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