David Cameron wants to block access to pornography unless people opt in, but is it possible to filter only the things you want?

David Cameron's government has laid out plans to crack down on internet pornography by forcing internet service providers to enable filters by default. In theory, those who leave the filter unchanged will not be able to access online material that is classified as inappropriate for those under the age of 18. "About time!" say those who fear the pervasiveness of adult material. "Censorship!" say those who fear encroaching state powers. But can it even work?

12 reasons why porn filters won't work

1. It's impossible to filter just what you want to

There is a tradeoff between failing to block inappropriate content and erroneously blocking harmless material which is educational, medical or artistic. A 2007 paper by the University of California, Berkeley, tested 15 combinations of internet content filters and filter settings. The most restrictive of those filters managed to block 91% of adult content – but it also mistakenly blocked 23% of "clean" webpages. The less restrictive filters had fewer errors but only managed to restrict access to 40% of material which was deemed inappropriate for children.

2. Filters can't cope with context and nuance

Filters that fail to distinguish between pornography and sites that provide advice on sexual health, sexuality and relationships could actually do more harm than good. ONS stats released on Thursday say 43% of people aged over 16 use the internet to seek health-related information – that figure has more than doubled since 2007 and will probably continue to rise.

3. You can't be clear about what you filter

It's virtually impossible to be transparent about internet filtering. There are 4.2m pornographic websites and 68m search engine requests to find those websites every day according to Internet Filter Review. Their statistics are likely to be guesstimates but they do show the sheer scale of a big chunk of our online world. Would the government publish a searchable database of every banned site? Would that list be updated regularly? With explanations about why each site was blocked? Probably not. That poses big problems for democratic principles such as oversight and accountability, especially when it's not clear why a site has been blocked. As the Internet Policy Review points out: "HTTP error codes are used by some filtering regimes. These are commonly either a 404 ('file not found') or 403 ('access denied') code." Those messages can mean very different things to an internet user.

4. It's not as simple as blocking a website

As well as performance issues, questions have been raised about the scope of internet filters. Some types of filters are unable to block material exchanged through peer-to-peer networks such as instant messaging, streaming video or file-sharing programs.

5. Many children are more tech savvy than their parents

Children will simply change the settings. This is a criticism that Cameron may have pre-empted but did not necessarily address when he stated: "It should not be the case that technically literate children can just flick the filters off at the click of a mouse without anyone knowing … those filters can only be changed by the account holder, who has to be an adult." A US panel officially mandated by the Child Online Protection Act argued: "A child can simply guess the override security word/number set by a parent to switch a filter off for adult use (how many parents set their password to their birth date or license plate number)." Crudely put, a filter has to be simple enough for technologically feeble adults but difficult enough to stop a tech-savvy 17-year-old working out how to bypass it. Arguably, no such filter exists.

6. There will always be ways round

Proxy servers can be used to bypass the filters. And there are countless tutorials online about how to use these servers. Attempts to block search results for "how do I bypass the filter?" would also need to block search results for "how can I see search results for the question how do I bypass the filter?".

7. For parents it may provide a dangerous illusion of safety

Filters could send the wrong message to parents that their children are unable to access inappropriate content online. Adults would have a false impression of security and fail to take steps to inform their children about online risks.

8. Filtering at router-level is inflexible

The proposals are unworkable because they are not device specific. A household may want one level of internet filtering for its shared computer, and another entirely for its mobile phones.

9. It sets a precedent for restricting legal content

Some of the content which Cameron proposes to filter is legal. Campaigners such as the Open Rights Group believe that excluding legal material online amounts to censorship and that it sets a dangerous precedent for other groups in the UK (and other governments abroad) who have an interest in suppressing information. Some specific cases illustrate the point well. The Clinton administration might have appreciated mandatory pornography filters at the time of the Monica Lewinsky scandal, for example.

10. It's not cost effective

High-profile technology advisers such as the Wikipedia co-founder Jimmy Wales have claimed that the plans are a "ridiculous idea" because they waste money that would be better spent on bolstering police resources to deal with online crime. Even the 2008 report by the Australian Communications and Media Authority admitted in its conclusion that it had not been able to "consider the balance of costs and benefits" associated with implementing filters.

11. It hasn't happened yet

We've been here before. Cameron "unveiled" almost identical plans in October 2011. That would suggest at best that these plans are difficult to implement and at worst that they are distraction policies, designed to deflect attention from more detrimental news items. Cynics will be quick to point out that the plans were unveiled the same day that a group of MPs released a report saying that official migration statistics were dangerously inaccurate and "could be out by tens of thousands". It was also the day that the "go home" poster campaign designed to target illegal immigrants attracted criticism.

12. And finally it's a gimmick rather than a solution

Internet filters obscure the real debate about the psychological, social and physical effects of online porn.

And three reasons why it could work

1. It's already happening

Filters are already viable, as shown by the commercial success of thousands of companies that provide this service both for the home and for the workplace. International Data Corporation (IDC) believes that the market for web security is rapidly growing – in 2011 it was worth $2.3bn (£1.5bn).

2. Other countries have systems to restrict child abuse content

Internet filtering is nothing new. Finland, Sweden, Norway, Denmark, the UK, Italy, the US, Canada and New Zealand have all established self- or co-regulatory blocking systems to prevent access to child abuse content.

3. And some research suggests filters can be made to work

Internet filters can be extremely accurate at blocking inappropriate content. That was one of the conclusions of a report by the Australian Communications and Media Authority in 2008 which found that filters had become considerably more sophisticated than their last trial of these in 2005. Subsequent plans to implement a mandatory filter in Australia were however mired in controversy and eventually abandoned.

Think we've missed any of the evidence for or against the effectiveness of porn filters? Post a comment below and let us know.

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