  The radiative decay of neutral fermions has been studied for decades but $CP$
violation induced within such a paradigm has evaded attention. $CP$ violation
in these processes can produce an asymmetry between circularly polarised
directions of the radiated photons and produces an important source of net
circular polarisation in particle and astroparticle physics observables. The
results presented in this work outlines the general connection between $CP$
violation and circular polarisation for both Dirac and Majorana fermions and
can be used for any class of models that produce such radiative decays. The
total $CP$ violation is calculated based on a widely studied Yukawa interaction
considered in both active and sterile neutrino radiative decay scenarios as
well as searches for dark matter via direct detection and collider signatures.
Finally, the phenomenological implications of the formalism on keV sterile
neutrino decay, leptogenesis-induced right-handed neutrino radiative decay and
IceCube-driven heavy dark matter decay are discussed.
