  We explore the systematics of odd-even mass staggering with a view to
identifying the physical mechanisms responsible. The BCS pairing and mean field
contributions have A- and number parity dependencies which can help disentangle
the different contributions. This motivates the two-term parametrization c_1 +
c_2/A as a theoretically based alternative to the inverse power form
traditionally used to fit odd-even mass differences. Assuming that the
A-dependence of the BCS pairing is weak, we find that mean-field contributions
are dominant below mass number A~40 while BCS pairing dominates in heavier
nuclei.
