  Interacting massive fields with m > d H/2 in d+1 dimensional de Sitter space
are fundamentally unstable. Scalar fields in this mass range can decay to
themselves. This process (which is kinematically forbidden in Minkowski space)
can lead to an important change to the propagator and the physics of these
fields. We compute this decay rate by doing a 1-loop computation for a massive
scalar field with a cubic interaction. We resum the 1-loop result by
consistently solving the Schwinger-Dyson equations. We also perform an explicit
resummation of all chain graphs in the case of the retarded propagator. The
decay rate is exponentially suppressed for large m/H and the flat space answer
(vanishing decay rate) is reproduced in that limit.
