"All the monitors observed during the study would connect to file-sharers and verify that they were running the BitTorrent software, but they would not actually collect any of the files being shared, " he said.
The Motion Picture Association of America is already working on its own economic study of the harmful effects of file sharing, to be completed this autumn.
The study also revealed that so-called blocklists, used by some illegal file-sharers to prevent monitors from connecting to their computers, might not be much use.