  Nonzero neutrino masses imply the existence of degrees of freedom and
interactions beyond those in the Standard Model. A powerful indicator of what
these might be is the nature of the massive neutrinos: Dirac fermions versus
Majorana fermions. While addressing the nature of neutrinos is often associated
with searches for lepton-number violation, there are several other features
that distinguish Majorana from Dirac fermions. Here, we compute in great detail
the kinematics of the daughters of the decays into charged-leptons and
neutrinos of hypothetical heavy neutral leptons at rest. We allow for the decay
to be mediated by the most general four-fermion interaction Lagrangian. We
demonstrate, for example, that when the daughter charged-leptons have the same
flavor or the detector is insensitive to their charges, polarized
Majorana-fermion decays have zero forward/backward asymmetry in the direction
of the outgoing neutrino (relative to the parent spin), whereas Dirac-fermion
decays can have large asymmetries. Going beyond studying forward/backward
asymmetries, we also explore the fully-differential width of the three-body
decays. It contains a wealth of information not only about the nature of the
new fermions but also the nature of the interactions behind their decays.
