According to a study carried out at Carlos III of Madrid University, a very small number of users are responsible for uploading a significant majority of the content on torrent sites.
The research looked at 55,000 torrent files on the popular sites Mininova and The Pirate Bay and found that two thirds - or 66 per cent - were uploaded by a group of around 100 individuals. The torrents uploaded by these people also accounted for 75 per cent of the downloads from these sites.
Given this information, the researchers tried to determine why these individuals might be carrying out these seemingly altruistic acts. The findings split the uploaders into two groups: fake publishers and top publishers. The fake publishers uploaded fake files that were often an attempt to infect an unwitting user's computer with malicious software. Alternatively, they could be members of groups trying to fight piracy by uploading dummy files.
The top publishers, on the other hand, push vast quantities of content onto the torrent networks in an effort to make money from either advertising or, to a lesser extent, VIP subscriptions.
Drawing on all of the data, the researchers were able to conclude that torrent uploaders are largely motivated by profit, rather than altruism. They also determined that torrent networks - and by extension, illegal downloading - could be brought to their knees by simply stopping these few individuals.
Of course, it would never be so easy. Apparently they tend to rent private servers to upload from, keeping their identity almost totally anonymous.
More details can be found here.