A new academic study sheds light on the question of how piracy affects manga sales, with some surprising results, as summarized by Torrent Freak. The study, by Professor Tatsui Tanaka of the Faculty of Economics at Keio University, compared sales of manga as availability of pirated copies varied due to a massive takedown program by the Japanese anti-piracy group CODA (Content Overseas Distribution Association).
The key variable that determined the impact of reducing the availability of pirated manga was whether a series was ongoing or completed, the study found. Series that were completed sold better when there were pirated copies available, presumably because people became aware they existed because of the pirated copies.
The reverse was true for ongoing series: sales declined when there were pirated copies available.
So it turns out that both of the arguments made on opposite sides of the piracy debate were true to some extent. Scanlators that defended the pirate sites and claimed that they amounted to marketing for sales of manga were correct for older series; but publishers, retailers, and others that deplored the pirate sites, arguing that they were hurting sales, were correct for ongoing series.
Tanaka suggests that publishers focus their takedown efforts on pirated copies of ongoing manga, leaving older series up on pirate sites to promote sales of legitimate versions.