  Surfaces of experimental masses of even-even and odd-odd nuclei exhibit a
sharp slope discontinuity at N=Z. This cusp (Wigner energy), reflecting an
additional binding in nuclei with neutrons and protons occupying the same shell
model orbitals, is usually attributed to neutron-proton pairing correlations. A
method is developed to extract the Wigner term from experimental data. Both
empirical arguments and shell-model calculations suggest that the Wigner term
can be traced back to the isospin T=0 part of nuclear interaction. Our
calculations reveal the rather complex mechanism responsible for the nuclear
binding around the N=Z line. In particular, we find that the Wigner term cannot
be solely explained in terms of correlations between the neutron-proton J=1,
T=0 (deuteron-like) pairs.
