  We measure the complexity of songs in the Million Song Dataset (MSD) in terms
of pitch, timbre, loudness, and rhythm to investigate their evolution from 1960
to 2010. By comparing the Billboard Hot 100 with random samples, we find that
the complexity of popular songs tends to be more narrowly distributed around
the mean, supporting the idea of an inverted U-shaped relationship between
complexity and hedonistic value. We then examine the temporal evolution of
complexity, reporting consistent changes across decades, such as a decrease in
average loudness complexity since the 1960s, and an increase in timbre
complexity overall but not for popular songs. We also show, in contrast to
claims that popular songs sound more alike over time, that they are not more
similar than they were 50 years ago in terms of pitch or rhythm, although
similarity in timbre shows distinctive patterns across eras and similarity in
loudness has been increasing. Finally, we show that musical genres can be
differentiated by their distinctive complexity profiles.
