  Considering the scheme with mixing of three neutrinos and a mass hierarchy
that can accommodate the results of solar and atmospheric neutrino experiments,
it is shown that the results of solar neutrino experiments imply a lower bound
for the effective Majorana mass in neutrinoless double-beta decay, under the
natural assumptions that massive neutrinos are Majorana particles and there are
no unlikely fine-tuned cancellations among the contributions of the different
neutrino masses. Considering the four-neutrino schemes that can accommodate
also the results of the LSND experiment, it is shown that only one of them is
compatible with the results of neutrinoless double-beta decay experiments and
with the measurement of the abundances of primordial elements produced in
Big-Bang Nucleosynthesis. It is shown that in this scheme, under the
assumptions that massive neutrinos are Majorana particles and there are no
cancellations among the contributions of the different neutrino masses, the
results of the LSND experiment imply a lower bound for the effective Majorana
mass in neutrinoless double-beta decay.
