  We show that, in four-dimensional spacetimes with an arbitrary Einstein
metric, with and without a cosmological constant, perturbative dynamical
degrees of freedom in generic quadratic-curvature gravity can be decoupled into
massless and massive parts. The massive part has the structure identical to,
modulo the over-all sign, the non-Fierz-Pauli-type massive gravity, and a
further decomposition into the spin-2 and spin-0 sectors can be done. The
equivalence at the level of equations of motion allows us to translate various
observational bounds on the mass of extra fields into constraints on the
coupling constants in quadratic curvature gravity. We find that the
Weyl-squared term is confronting two apparently contradicting constraints on
massive spin-2 fields from the inverse-square law experiments and observations
of spinning black holes.

General Relativity and Quantum Cosmology;High Energy Physics - Phenomenology;High Energy Physics - Theory