Magellan Media Partners: “The Impact of Piracy“:

Both pirated and un-pirated titles showed similar growth in sales in the first few weeks after a title is published, followed by a decline after peak. Average sales for unpirated content start higher and peak later, although this may reflect the specific nature of titles in a small sample.
The primary difference between sales of pirated and unpirated content appeared in weeks 19 through 25, when sales for pirated content peaked a second time at a level higher than that seen in the first, sell-in period. This second peak followed the time (19 weeks) at which the average pirated O’Reilly front-list title was first seeded on a P2P site.
We stress that this is correlation, not causality, but the difference in the sales profile is notable and persists even when using rolling averages.