In the U.S. roughly half a million people have been sued for sharing copyrighted files in recent years, but filing of mass-lawsuits is not getting easier. A federal judge in Iowa has just issued a key order which makes mass-BitTorrent piracy lawsuits virtually impossible. The judge ruled that copyright holders can't join multiple defendants in one suit, since there is no proof that they shared files with each other.

In recent years hundreds of thousands of Internet subscribers have been sued for downloading and sharing copyrighted material in the United States.

Most of these defendants have been grouped together in large cases, which makes it cheap for the copyright holder to obtain the personal details of the alleged infringers. Instead of paying filing fees for each defendant, they pay a single fee by suing dozens, hundreds or thousands at once.

These so-called mass-BitTorrent lawsuits are possible because copyright holders argue that the file-sharers acted in concert. They all downloaded the same torrent file and joined the same swarm, they reason.

In a set of recent rulings involving the independent movies Killer Joe, Sibling and The Company You Keep, Federal Judge Stephanie Rose disagrees, as she actually took the time to understand how BitTorrent works.

In order to join multiple defendants in one lawsuit a copyright holder has to make it clear that they were involved in the same series of transactions. In other words, it has to be likely that the alleged pirates traded files with each other.

In the cases in question, there were weeks or even months between the time the first and last defendant was spotted sharing the film. This makes it very unlikely that all defendants did in fact share files with each other.

“In the Killer Joe cases, the January defendants would have to be connected to the Internet and still actively distributing data through the BitTorrent client approximately three months later to be involved in the same transaction as the April defendants, which is implausible at best,” the Judge writes.

Even more so, even if the defendants were found to share the file on the same day, that would still provide no evidence that they shared bits and pieces with each other.

“Even in all five cases where Doe defendants allegedly have ‘hit dates’ on the same day and close in time, there is no showing that the earlier defendants were still connected to the Internet and actively distributing data through the BitTorrent client at the same time as the later defendants,” she adds.

The copyright holders argued that alleged pirates can be joined in one lawsuit because they shared a torrent file with an identical SHA-1 hash. However, the Judge disagrees and notes that the hash provides no evidence that the defendants actually interacted with each other.

“Although each plaintiff has alleged that the defendants in each case were in the same swarm based on the same hash value, participation in a specific swarm is too imprecise a factor absent additional information relating to the alleged copyright infringement to support joinder,” Judge Rose writes.

“Any ‘pieces’ of the work copied or uploaded by any individual Doe may have gone to any other Doe, but may instead have gone to any of the potentially thousands of others who participated in a given swarm and are not in this case,” she adds.

Based on the above, Judge Rose concludes that all five mass-BitTorrent lawsuits should be limited to one defendant. The cases of all others are therefore dismissed.

Since copyright holders can’t easily prove that all defendants shared bits and pieces with each other, it is unlikely that the movie studios will succeed at filing cheap mass-lawsuits before this judge.

The ruling is definitely good news for the thousands of defendants who will be put in a similar position in the future. While there are no guarantees that other judges will reach the same conclusion, it provides defendants with additional ammunition to fight these cases.