On the old ten-inch shellac disc, the recording duration was limited to slightly over three minutes. As this was the first vehicle for popular music, all pop songs were abbreviated to meet this technical limitation. But curiously, when the long-play disc was introduced in 1948, the length of the average pop song did not increase in proportion. This suggests that some mysterious law concerning average attention span may have been inadvertently discovered by the older technology.

Schafer, Murray R, The Soundscape- Our Sonic E…, loc. 349