Studying the web's impact on sex offenders
In May 2011, Lincolnshire Police announced that it had broken up a web-based news group that sold and distributed images of child sex abuse.
A four-year investigation named Operation Alpine led to the closure of the site and resulted in 132 UK children being identified and safeguarded. In addition, 55 arrests have been made of people who ran and used the site. More arrests are expected.
The vast majority of the 200 UK suspects who accessed the illegal images were not previously known to the police.
It is a feature that crops up time and again during investigations of sites that peddle child sex abuse images, said Mick Moran, Interpol's co-ordinator on child abuse.
"The vast majority of people being arrested for possession of child sex abuse material are between 25-45, white and taxpayers," he said. "Those people are not normally the ones that come to the police's attention."
Mr Moran believes that is odd because the web's openness and lack of anonymity make it a poor choice for anyone breaking the law.
"From a law enforcement point of view and a sex offender point of view the web is not the ideal source of this material," said Mr Moran.
Established offenders prefer to get material they covet via more covert sources such as invitation-only discussion forums, peer-to-peer networks and dedicated file transfer sites, he explained.
When images of child sex abuse appear on the web there's usually a very obvious motive.
"The web is only used where there is an attempt to monetise this material," he said.
That view stands in contrast to the other places where established abusers trade content, not for cash, but on a like-for-like exchange basis.
In the UK, the Internet Watch Foundation shuts down websites where images of child sex abuse are being shared and offered for sale. In 2010, the IWF's annual report said that it knew of 14,602 sites peddling images. About 59 pages a day were added to that list.
Most of the reports about sites hosting the images come from members of the public that stumble across the material, said Emma Lowther, a spokesperson for the IWF.
Much of the material found online is simply copied from elsewhere, she explained.
"A lot of the content is recycled," she said. "As soon as an image is put up in one place it can be copied and replicated elsewhere."
The ease with which content can be copied and shared creates its own problems for police forces as they comb through it to identify victims, abusers and networks.
Operation Alpine netted more than one million images and 6,000 films. Other operations have produced similarly huge amounts of material.
In many cases it is far too much for any individual to look through and categorise, said Pelle Gara from technology firm Net Clean which helps some police forces catalogue what they find.
Analysing all the content was important, he said, because the charges brought against defendants depend on the severity of the images and films found.
Computers can do the work quickly by reducing images to a digital shorthand. They can compare what has been turned up in a recent raid to older material. The analysis can also aid investigations by teasing out links between sites and offenders.
"The pictures are the constant, not the sites they appear on," he said.
Letting computers do the analysis also means investigators and officers only have to look at content that has never been seen before.
That is good, said Mr Gara, because it reduces the number of images investigators have to look at. The psychological toll of viewing thousands of pictures or videos can be heavy.
While more experienced offenders shun the web when seeking images of child sex abuse, evidence is starting to accumulate about the broader effect it has.
"The web has hugely facilitated exposure to this type of material," said Mr Moran, particularly for a class of what is known as "naive" offenders who are only starting to seek out such content.
That may help to explain why highly professional, educated people that were not known before to police are being caught in possession of the material.
Law enforcement agencies and psychologists are debating the relationship between the availability of the material online and those who are interested in it.
Dr Ethel Quayle, a lecturer in clinical psychology at the University of Edinburgh said that studies have done little to settle the question of what was more important to them - the easy availability of images or a latent sexual attraction to minors.
What was clear, she said, was that the web fulfils many of its founding impulses for the communities of offenders that spring up online.
"The internet was always thought of in its early stages as really good learning environment," she said, "and to some extent that's what we also see if we look in some of the chat logs."
"A large amount of the content is learning from others how not to get caught," she said. Dr Quayle also believes that mixing in such a community helps to legitimise an interest in images of child sex abuse.
"Whatever else it does, no matter how large a step you take into that community, there's growing evidence that hanging out with the bad guys means you may do more," she said. "There's evidence to suggest that, but we are still a long way from knowing what that relationship is."
Dr Quayle said that it was unclear whether individuals would have begun offending, without access to the images.
But the figures are worrying. Although convictions for sexual offences against children are falling, the proportion of offences that relate to internet crimes against children are going up.
"It's a problem that's simply not going away," said Dr Quayle.