Is the single-state dominance realized in double-β-decay transitions?

In the single-state-dominance hypothesis (SSDH) the decay rate of the two-neutrino double-β decay to the final ground state is solely determined by virtual single-β-decay transitions via the $1^{+}$ ground state of the intermediate nucleus. A very important consequence the SSDH will be that some of nonaccelerator measurements of double-β-decay observables could be circumvented by single-β-decay measurements. To assess the validity of the SSDH, we have carried out a theoretical analysis of all double-β-decay transitions where the spin-parity of the ground-state of the intermediate nucleus is $1^{+}$. The calculations indicate that the double-electron emitters, as well as the less explored double-positron emitters, show evidence of the SSDH.