  We show that, within some modified gravity theories, such as the Palatini
models, the non-linear nature of the field equations implies that the usual
naive averaging procedure (replacing the microscopic energy-momentum by its
cosmological average) could be invalid. As a consequence, the relative motion
of particles in Palatini theories is actually indistinguishable from that
predicted by General Relativity. Moreover, there is no WEP violation. Our new
and most important result is that the cosmology and astrophysics, or put more
generally, the behaviours on macroscopic scales, predicted by these two
theories are the same, and as a result the naturalness problems associated with
the cosmological constant are not alleviated. Palatini gravity does however
predict alterations to the internal structure of particles and the particle
physics laws, e.g., corrections to the hydrogen energy levels. Measurements of
which place strong constraints on the properties of viable Palatini gravities.
