  We investigate purely leptonic decays of leptons $\l^{\prime-}\to
l^-\bar{\nu}_l\nu_{l^{\prime}}$ to distinguish Dirac or Majorana neutrinos. We
derive the differences of the decay width (and associated quantities) between
the two neutrino hypotheses by the effect of identical particles. Evidence from
experimental data makes the hypothesis of Majorana neutrinos very unlikely and
our results strongly favor the hypothesis of Dirac neutrinos (in the standard
three-neutrino theory). Moreover, our exploration for decay widths can be
extended to the polarization angles of leptons, which will inspire new
information for experimental and theoretical studies.
