The stability of the Hg2(2+) cation and related species is due to differential aggregation/solvation effects in the condensed phase. These are strongly modified by relativistic effects. Thus, relativity is responsible for the existence of Hg-Hg-bonded species, but only in the condensed phase, and the stability is not due to the relativistic strengthening of the metal-metal bond itself, as suggested earlier. Ab initio pseudopotential calculations at theoretical levels higher than previously reported show that relativistic effects clearly shift the equilibrium Hg2X2(g) reversible HgX2(g) + Hg(g) (X = F, Cl, H) to the right and not to the left. There is a considerably greater chance to find the corresponding Zn-Zn- or Cd-Cd-bonded species in the gas phase! In the condensed phase, differential aggregation or solvation effects favor the Hg(n)2+ cations : (a) The shift of the equilibrium to the right by the aggregation energy of the elemental metal is less pronounced for M = Hg than for M = Zn and Cd, very likely due to relativity. (b) The relativistic reduction of aggregation or solvation energies is larger for HgX2 species than for the corresponding Hg2X2 compounds. This is shown by calculations on molecular model systems, MCl2.H2O, M2Cl2.H2O, (MF2)2, and (M2F2)2, and by periodic Hartree-Fock calculations on solid Hg2F2 and HgF2.