Swedish Internet users love themselves some file-sharing—but not when their veil of anonymity can be stripped away.

The EU's Intellectual Property Rights Enforcement Directive (IPRED) requires all member states to adopt tougher protections against piracy and counterfeiting. Sweden's own implementation of the law went into effect on the first day of April and had an immediate effect; Internet traffic passing though the country's main exchange points fell nearly in half.

The main provision of Sweden's IPRED directive would not be specially controversial in the US; indeed, it's already the law here. Copyright holders who identify the IP address of someone sharing their material online can go to a court, which can force ISPs to turn over information about the subscriber using that IP address at the time in question. Should ISPs then choose to sue the person, they can do so.

The threat of unmasking appears to have slashed Internet traffic in Sweden. Netnod, which runs major Swedish Internet exchange points in Lule?, Sundsvall, Stockholm, Gothenburg, and Malm?, publishes data about the traffic passing through its facilities. Its graphs show that total traffic dropped from a peak of 190Gb/s on Tuesday, March 31 down to a peak of only 110Gb/s on Wednesday, when the new law went into effect.

Some declines were even more significant. Traffic at the Gothenburg site alone plummeted even further (as a percentage), dropping from 16Gb/s to 7Gb/s, and today's numbers appear to be even lower than yesterday's.

Of special note is the fact that traffic on March 31 was higher than the average at every site, suggesting a last-minute bingeing at the Internet's P2P buffet.

Will the effect last? Who knows. Some overall reduction seems likely, but it will probably be modest; countries where rightsholders can already unmask IP addresses certainly haven't managed to curtail files-sharing simply by threatening lawsuits. Even the RIAA's lawsuit campaign in the US failed to cut files-swapping substantially.

But the dramatic data will no doubt provide the impetus for rightsholders to push even harder for graduated response laws across the globe. One of the key points of such a campaign is stripping away the sense of anonymity; even in trials where the ISP passes along infringement notices but doesn't threaten disconnection, P2P usage has dropped. One UK study last year suggested that 70 percent of file-swappers would stop sharing copyrighted files after a simple notice that their activity had been detected.

Sweden's Internet number might well recover as the "novelty" of the new threat wears off and most Internet users remain unsued, but the data does seem to suggest that so long as people feel exposed, they will alter their behavior.

Of course, rather than stop sharing files—practically a national pastime in Sweden—Internet users may turn instead to services like The Pirate Bay's new "IPREDator" anonymous VPN.

Rightsholders are already putting the new law to use, as several book publishers have gone to court seeking information about the online sharing of their audiobooks.