  If we accept a paradigm that star formation is a self-similar, hierarchical
process, then the Salpeter slope of the IMF for high-mass stars can be simply
and elegantly explained as follows. If the instrinsic IMF at the smallest
scales follows a simple -2 power-law slope, then the steepening to the -2.35
Salpeter value results when the most massive stars cannot form in the
lowest-mass clumps of a cluster. It is stressed that this steepening MUST occur
if clusters form hierarchically from clumps, and the lowest-mass clumps can
form stars. This model is consistent with a variety of observations as well as
theoretical simulations.
