By Joan Smith, Chair of the Mayor of London's Violence Against Women and Girls Panel

Another man has joined the growing list of convicted murderers 'obsessed' with violent porn.

Nathan Matthews, 28, was found guilty yesterday of the brutal murder of his 16-year-old step-sister Becky Watts, whom he suffocated and stabbed after going to her home equipped with stun guns and handcuffs.

It's now emerged that he watched a film entitled ‘virgin teen gets raped in own house’ before attacking her.

Last month, a serious case review into the murder of 17-year-old Georgia Williams, a policeman’s daughter from Telford, highlighted failings by police and social services. Her killer, Jamie Reynolds, 23, had previously tried to strangle a girl in 2008 and went on to kill Georgia in 2013.

Reynolds had been obsessed with hanging, strangulation and necrophilia from the age of 15. His collection included 16,800 pornographic images and 72 videos of sexual violence; he also doctored photos of girls he knew to make them look as though they had been hanged. His step-father told the Justice Liaison Service he had had discovered Reynolds viewing images of women being strangled; he was advised to go to the police but the force failed to act on his report.

Georgia Williams Credit: EPA

Other cases involving violent porn include the murder of Joanna Yeates by Vincent Tabak, who strangled the landscape architect in 2010 while they we living in adjoining flats in a house in Bristol. The murder attracted huge attention and it later emerged that Tabak had violent images on his laptop, including videos of blonde women being strangled. Earlier this year, Tabak admitted four charges of possessing indecent images of children on his computer.

Two other men, Mark Bridger and Stuart Hazell, are serving long prison sentences after carrying out lethal attacks on children. Bridger killed five-year-old April Jones in Wales, while Hazell murdered his step-grand-daughter Tia Sharp, aged 12, in south London. Bridger spent a great deal of his time viewing violent porn, including 65 images that met the standard for criminal prosecution. Hazell had a history of viewing paedophile material online and possessed memory cards showing indecent pictures of children and images of bestiality. He also secretly filmed Tia before the murder.

Vincent Tabak was found guilty of murdering Joanna Yeates

These cases reflect a hugely disturbing trend, which is the apparent ‘normalisation’ of watching extremely violent porn among some groups of men.

Thirty years ago, ‘hardcore’ porn consisted of unpleasant magazines and videos that had to be obtained from specialist suppliers, with some risk to both sides. Now it has not only moved online, where teenage boys and adult men are able to access it, but it has become even more graphic and violent.

People who associate the word ‘porn’ with Playboy magazine would be shocked to the core if they knew how brutal this stuff is, or how much of it explicitly involves sexual violence. Masses of material is available in categories such as ‘rape porn’, while individuals who want it can find videos showing women being strangled. It is suspected that some of this material is non-consensual, created by traffickers who make porn videos as a side-line to their main business of forced prostitution.

April Jones and Mark Bridger Credit: PA

"Do men become violent after watching porn or are violent men drawn to watching it?"

This explosion in violent porn poses challenges for the criminal justice system.

Possession of some material, such as pornography involving children, is a criminal offence and the police try to catch consumers through their use of websites and credit cards. Obviously those men should be charged and taken to court, where the publicity may have a deterrent effect, while female friends and partners are at least made aware of a potential risk.

In the past, researchers have been wary of making assumptions about the impact of porn even on frequent users, pointing out the big conundrum: do men become violent after watching porn or are violent men drawn to watching it?

In a sense, the change in the type of pornographic material available makes that question redundant, because the link between pornography and violence is now so much more explicit.

Most disturbingly, in all the cases mentioned, there is an undeniable similarity between the videos viewed by the offenders and their crimes. They seem to have progressed from viewing unpleasant material that fed their fantasies to treating it as a kind of rehearsal for the murders they went on to commit.

The notion that watching women being tied up, raped, stabbed and strangled on a regular basis has no impact defies common sense. Men who do this recreationally are not likely to emerge with an enhanced respect for women; aside from the possibility of copycat attacks in real life, the desensitising effect is worrying enough.

Stuart Hazell has been charged with Tia Sharp's murder Credit: Getty images

"The notion that watching women being tied up, raped, stabbed and strangled on a regular basis has no impact defies common sense."

That’s why the subject should be addressed in schools, with teenage boys and girls being told that this is not healthy or acceptable behaviour.

At the same time, when someone comes to the attention of the police, the regular viewing of violent porn should in itself be regarded as a risk factor.

However liberal we might want to be, it’s hard to deny that an individual who collects stabbing videos might pose a danger to women and girls in his social circle. Porn has changed, and our assumptions need to change with it.