Birgitte Andersen and Marion Frenz wrote a study on how file sharing effects CD sales in Canada. Their findings are rather frustrating to those who want to generally blame file sharers as scapegoats, as this does not work, at least not in Canada.
In a nutshell:
This study measures the extent to which P2P file-sharing activities act as substitutes or complements to music purchases in markets for CDs. The paper breaks with the mainstream economics approach which dominates the music file-sharing discussion. Whereas such models assume relationships at the micro level (e.g. between file-sharing and purchases) based on observations made at the macro level, our evolutionary economics approach measures the direct effects using micro data representative of the Canadian population. The behavioral incentives underpinning free music downloading, novel to this paper, are the multiple effects of: ‘unwillingness to pay’ (market substitution), ‘hear before buying’ (market creation), ‘not wanting to buy a whole album’ (market segmentation), and ‘not available in the CD format or on electronic pay-sites’ (market creation). Although the two first mentioned incentives significantly influence CD album purchases—i.e. there is a negative and significant market substitution effect and a positive and significant market creation effect—on the whole, these two effects ‘cancel’ one another out, leading to no association between the number of P2P files downloaded and CD album sales.
get the full study here: http://www.springerlink.com/content/d3h27828tx887556/fulltext.pdf