File-Sharing and Copyright, by Felix Oberholzer-Gee and Koleman Strumpf, Harvard Business School, Working Paper 09-132

From the Introduction:

The advent of file-sharing technology has allowed consumers to copy music, books, video games and other protected works on an unprecedented scale at minimal cost. In this essay, we ask whether the new technology has undermined the incentives of authors and entertainment companies to create, market and distribute new works. While the empirical evidence of the effect of file sharing on sales is mixed, many studies conclude that music piracy can perhaps explain as much as one fifth of the recent decline in industry sales. A displacement of sales alone, however, is not sufficient to conclude that authors have weaker incentives to create new works. File sharing also influences the markets for concerts, electronics and communications infrastructure. For example, the technology increased concert prices, enticing artists to tour more often and, ultimately, raising their overall income.

Data on the supply of new works are consistent with our argument that file sharing did not discourage authors and publishers.

-> Michael Geist's summary