  Background: The presence of nuclear ground states with stable
reflection-asymmetric shapes is supported by rich experimental evidence.
Theoretical surveys of odd-multipolarity deformations predict the existence of
pear-shaped isotopes in several fairly localized regions of the nuclear
landscape in the vicinity of near-lying single-particle shells with
$\Delta\ell=\Delta j=3$. Purpose: We analyze the role of isoscalar, isovector,
neutron-proton, neutron-neutron, and proton-proton multipole interaction
energies in inducing the onset of reflection-asymmetric ground-state
deformations. Methods: The calculations are performed in the framework of axial
reflection-asymmetric Hartree-Fock-Bogoliubov theory using two Skyrme energy
density functionals and density-dependent pairing force. Results: We show that
reflection-asymmetric ground-state shapes of atomic nuclei are driven by the
odd-multipolarity neutron-proton (or isoscalar) part of the nuclear interaction
energy. This result is consistent with the particle-vibration picture, in which
the main driver of octupole instability is the isoscalar octupole-octupole
interaction giving rise to large $E3$ polarizability.
