  The coefficient of the dimensionally regularized two-loop R^3 divergence of
(nonsupersymmetric) gravity theories has recently been shown to change when
non-dynamical three forms are added to the theory, or when a pseudo-scalar is
replaced by the anti-symmetric two-form field to which it is dual. This
phenomenon involves evanescent operators, whose matrix elements vanish in four
dimensions, including the Gauss-Bonnet operator which is also connected to the
trace anomaly. On the other hand, these effects appear to have no physical
consequences in renormalized scattering processes. In particular, the
dependence of the two-loop four-graviton scattering amplitude on the
renormalization scale is simple. In this paper, we explain this result for any
minimally-coupled massless gravity theory with renormalizable matter
interactions by using unitarity cuts in four dimensions and never invoking
evanescent operators.
