Adam Smith, a loyal MR reader (yes that is his name), writes to me:
I had a very MResque thought today I wanted to share with you. Why are the typical lengths of albums across different music genres so different? In particular, I was thinking most of my rap albums are at least over the hour mark and many run all the way up to the 80-minute maximum. They're usually packed with intros, skits, and lots of 5 minute tracks that have extended intro and outro instrumental beat only sequences. My metal albums, on the other hand, have an average run length of no more than 40 mins. Most albums are between 8 and 10 tracks with little in the way of tangential material. These different run-times show up in other places too. For example, my older jazz albums (i.e. Kind of Blue, Time Out, Blue Train) typically run around 45 mins with a half dozen or so tracks yet my newer jazz albums like MMW's The Dropper run almost the whole 80 mins. Also, prog. rock bands tend to produce much longer albums than garage rock bands. Even adjusting for the fact that prog bands emphasize longer musical passages, they could compensate by just having fewer songs or garage rock bands could just have twice as many (like the White Stripes did on their first album).
Is there a relative price argument for these differences? Or even signaling? Perhaps there is a rat race among rappers to signal they're capable of coming up with enough material to fill out the maximum length, even if it includes lots of filler. Perhaps the recording costs are lower as instrumentation relies so heavily on sampling. Maybe metal runs into diminishing returns after 30 mins or so where the listener becomes numb to the intensity.
I'll offer a few points:
1. The average career of a rapper is short. A long CD increases the chance that something will "stick" and the rapper won't get too many other chances to try.
2. Some metal bands develop great loyalty among their followers and achieve durable franchises. That gives them a lower discount rate and they are more inclined to save up material for the future. Plus they are marketing an overall sound — rather than clever particular innovations — and if the first forty (five?) minutes don't convince you nothing will. Rap songs probably have a higher individual variance.
3. Many older albums are short for technological reasons, plus the albums were due in relatively rapid succession for contractual reasons. In the 1960s there was lots of technological advance in music, so if you sat on the sidelines for a few years you could become obsolete.
4. It is relatively easy for a contemporary jazz artist to tack on additional improvisations and he can choose standard compositions if necessary. Other forms of popular music cannot expand quantity so easily without hitting a wall in terms of quality. One prediction here is that "compositional jazz" albums should be shorter in average length than albums of jazz improvisation, contemporary that is.
5. If you wanted a somewhat strained explanation, you could argue that the longer CD is a more bundled product and it will make economic sense as a form of price discrimination, the more varied the valuations of the audience. This would require that rap CD buyers have a higher variance of marginal valuation.