These two lawsuits put the fear of God, so to speak, into record companies, who began realizing that not only were records from their catalogs getting sampled but they were increasingly signing artists who made heavy use of samples. The industry decision was to enforce a rule where, if you release a record on a major label, you have to clear every sample you use.

So say I want to legally license a sample. What do I have to do?

To legally sample a recording you have to negotiate a separate sample clearance fee with two different rights-holders: whoever owns the sound recording (the actual sound that's been fixed to magnetic tape, CD, etc.) and the song publisher (who owns rights to the underlying melody and lyrics). This takes a lot of money and time. For well-known songs, licensing fees can be very expensive—and sometimes rights-holders won't agree to a sample clearance for any price.

But it gets way more complicated when you start sampling songs that contain samples, which is increasingly the case today. If you wanted to sample, say, "Fight the Power" by Public Enemy—well, that song contains 20 samples. You'd have to get permission from Def Jam, which owns the sound recording rights, and then Public Enemy's song publisher. Then you'd have to go to the other 20 song publishers and get permission to use the song—it creates kind of a domino effect. This licensing logjam is only going to get worse and worse and worse as people increasingly sample the recent past, since that recent past is already a collage. It just becomes impossible to do all these clearances.

You suggest two influential hip hop releases—the Beastie Boys' Paul's Boutique and Public Enemy's Fear of a Black Planet—would be financially and bureaucratically impossible to release today, due to their heavy sampling.

Right. This is something people have been saying for a long time. My co-author Peter DiCola and I were able to do some economic modeling to test the hypothesis.

We figured out—song by song, sample by sample—how much it would cost to release each record. Sticking with the example of Paul's Boutique: there are about 2.5 million units sold of that record. Incidentally, a lot of the samples on Paul's Boutique actually were cleared—but they were cleared at a time, 1989, when the industry didn't really see the value of sampling yet, so the rates for copyright clearances were much lower. Today, the rates they'd have to pay would make it impossible. Based on the number and type of samples in that record, Peter figured out that Capitol Records would lose 20 million dollars on a record that sold 2.5 million units. Fear of a Black Planet is similar.

Artists who want to appropriate have one major tool in their defense: the doctrine of fair use. What kind of sampling falls under this protection?

To determine between fair use and actual copyright infringement, judges and juries look at a variety of factors, such as: the length of the use, the purpose of the use, and more generally, whether the use is "transformative." A transformative use basically refers to a quotation in which you've changed the context, or altered the original sufficiently to transform it into something new. This concept has played an increasingly important role in fair use decisions of the past 20 years by courts.