  Musical scales are used throughout the world, but the question of how they
evolved remains open. Some suggest that scales based on the harmonic series are
inherently pleasant, while others propose that scales are chosen that are easy
to communicate. However, testing these theories has been hindered by the
sparseness of empirical evidence. Here, we assimilate data from diverse
ethnomusicological sources into a cross-cultural database of scales. We
generate populations of scales based on multiple theories and assess their
similarity to empirical distributions from the database. Most scales tend to
include intervals which are close in size to perfect fifths (``imperfect
fifths''), and packing arguments explain the salient features of the
distributions. Scales are also preferred if their intervals are compressible,
which may facilitate efficient communication and memory of melodies. While
scales appear to evolve according to various selection pressures, the simplest
h imperfect-fifths packing model best fits the empirical data.
