It is shown that the anomalously large double binding energy differences for even-even N = Z nuclei are a consequence of Wigner's SU(4) symmetry. These, and similar quantities for odd-mass and odd-odd nuclei, provide a simple and distinct signature of this symmetry in N≃Z nuclei

A universal, physically intuitive description of collectivity in heavy nuclei is obtained in terms of the parameter P =N p N n / (Np+N n ) which gives the average number of interactions of each valence nucleon with those of the other type. Regardless of mass region, deformation sets in when P =P C nt ~ 4-5. This, in turn, gives a rule that medium-mass and heavy nuclei with effective N p or N n < 4 cannot be deformed. An interpretation in terms of the monopole and quadrupole p-n interactions is discussed.PACS numbers: 21.60.Ev