On his Freedom to Tinker blog, Princeton computer science professor Edward Felten presents his "Grand Unified Theory of File Sharing," which attempts to explain discrepancies in various studies comparing file sharing activities with CD sales:

First, let's review the three main results that have to be explained.
Survey-based studies, which ask people whether they use the Internet, whether (and how much) they use filesharing, and how many CDs they buy, find that people who fileshare buy fewer CDs.

The recent econometric study by Oberholzer and Strumpf, based on per-album time-series data on filesharing activity, CD sales, and other factors, found that filesharing has little or no effect on CD sales.

Eric Boorstin's study found, controlling for differences in personal income, that there is a strong positive correlation between Internet usage and CD purchasing. This held true for all age groups, except the 15-24 group, for whom Internet usage correlates negatively with CD purchasing.

....The Grand Unified Theory explains the study results by breaking down the users of filesharing into two subpopulations, which I will call Free-riders and Samplers.

Free-riders are generally young. They have few if any moral qualms about filesharing, and they tend to assume that others feel the same way. They use filesharing to accumulate libraries of music, as an alternative to buying CDs.

Samplers are generally older and more risk-averse. They are highly engaged with cultural products of all sorts. They are morally conflicted about filesharing, and use it mostly to download songs that either aren't for sale, or that they don't value enough to pay for. They buy music that they really like, and filesharing causes them to find more music they like, so it tends to increase their CD purchases.

Now let's look at how the theory explains the studies' results.

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