Filtering all Internet traffic to look for signs of illicit file-swapping has been a hugely controversial idea, criticized on grounds of privacy, efficacy, cost, and long-standing "safe harbor" principles that apply to network operators such as ISPs and phone companies. But a new study out from UK media lawyers Wiggin suggests that, if it works, such filtering could actually curtail "digital piracy" by 70 percent.

The finding is of special relevance in the UK, where some form of ISP filtering is currently being considered as a way to deal with the illegal trading of copyrighted material. ISPs and content owners are engaged in a voluntary negotiation over how to address the problem, but the UK government has indicated that it will legislate in April 2009 if no agreement is reached by then. The government appears to be planning something similar to the French system, which is developing a "three strikes" approach to notifying and then blocking offenders.

Wiggin commissioned the 2008 Digital Entertainment Survey, which found that 70 percent of all people polled said they would stop illegally sharing files if their ISP notified them in some way that it had detected the practice. When broken down by age group, an unexpected trend emerges: teenagers are generally more likely to change their behavior than older Internet users.

The survey showed that people would also stop sharing files if they felt that the chance of being prosecuted was higher, but that 68 percent of all users think it is "very unlikely I will be caught." Even if more stories about P2P prosecutions find their way into the mainstream press, half of all users said they would continue to share files because they perceive the total risk to be quite small. A notice system that required only a letter or a phone call instead of a full-blown legal proceeding could clearly operate more quickly and more widely, and seems to have the best chance of altering behavior.

Whether ISP filtering can work, and can do so while avoiding false positives and maintaining user privacy, remains an open question, and there's still debate over the idea that ISPs should (as a matter of policy) become enforcers of the copyrights for a specific industry segment. Still, the new survey suggests that if these problems can somehow be overcome, ISP notification might well have a better chance of changing behavior than legal action alone.