David Cameron’s announcement of a crackdown on online pornography aims to tackle an issue some believe is having a corrosive effect, not just on children, but society more generally.

There are two issues under the microscope: images of child sexual abuse, or “child pornography”, and adult pornography.

Even though the prime minister is addressing both in today’s speech, they are very separate fields. Child pornography is probably the only subject about which there is genuine consensus across the internet. From hackers to chief executives, everyone agrees that child porn is wrong (with the exception of the tiny minority of paedophiles interested in accessing it).

Internet companies already work to prevent access to child porn – internet service providers (ISPs) like BT, Sky and Virgin Media block websites hosting this content. When Google’s powerful web crawling software finds such images, it reports the addresses to the Internet Watch Foundation, to which many companies contribute money.

22 onlineporn g w Big questions over Camerons plans to combat internet porn

Mr Cameron wants them to go further, suggesting that search engines should simply refuse to provide search results for certain blacklisted terms.

“There are some searches which are so abhorrent and where they can be no doubt whatsoever about the sick and malevolent intent of the searcher that there should be no search results returned at all,” he said.

There are two problems here: firstly, is it really possible to discern the intent from the search term (for example, is a search for “child porn” aiming to find the actual content, or just research material about it?).

Secondly, it is unlikely that paedophiles are turning to mainstream search engines to find this content. The exchange of these images takes place instead on niche chat forums, and through the so-called dark web, which can only be accessed by using special anonymising software.

On legal, adult pornography, the prime minister is trying to address concerns (from the likes of Claire Perry MP, his special adviser on preventing the commercialisation and sexualisation of childhood) that the easy availability of porn online is damaging to children’s upbringing.

He wants ISPs to introduce an opt-in system, so that if a customer wants access to porn they must actively choose this. That’s a shift from the ISPs’ preference, which is for porn to be available by default, but for customers to be able to opt out of being able to see it.

The ISPs do not see it as their job to police what their customers do online – they see it as the thin end of a very big wedge which will eventually see them held accountable for their customers’ internet use. They also argue that automated systems mean families will become complacent about teaching their children to use the internet.

There’s also a huge question mark over how such automated opt-in systems work. Some sites are clearly pornographic, but what about the work of Spencer Tunick, whose photographs of massed nudes might fall foul of the restrictions?

Many pages on the blogging site Tumblr contain pornographic content. Automated systems still struggle to work out which pages to block.

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