Unitarity in Higher-Derivative Quantum Gravity

A Euclidean lattice formulation is presented of the general fourth-order gravitational action involving ${R}_{\ensuremath{\mu}\ensuremath{\nu}}^{2}$ terms. This lattice theory is bounded and reflection positive. The existence of a positive Hilbert space and Hamiltonian, and hence unitarity, follow then from the standard nonperturbative Osterwalder-Schrader construction.