  The origin of the dominance of prolate shapes over oblate ones of the ground
states of atomic nuclei is investigated with the Nilsson-Strutinsky method. The
number of prolate nuclei among all the deformed even-even nuclei is calculated
as a function of the strengths of the spin-orbit and the l^2 terms of the
Nilsson potential. The latter simulates a square-well like radial profile of
the mean potential. The ratio of prolate nuclei is 86% with the standard
strengths corresponding to the actual atomic nuclei. By weakening the
spin-orbit potential, the ratio oscillates strongly, having a local minimum
value of 45% with a half of the standard strength and a local maximum value of
78% without the spin-orbit potential.
