  We propose the almost-geodesic motion of self-gravitating test bodies as a
possible selection rule among metric theories of gravity. Starting from a
heuristic statement, the "gravitational weak equivalence principle", we build a
formal, operative test able to probe the validity of the principle for any
metric theory of gravity, in an arbitrary number of spacetime dimensions. We
show that, if the theory admits a well-posed variational formulation, this test
singles out only the purely metric theories of gravity. This conclusion
reproduces known results in the cases of general relativity (also with a
cosmological constant term), and scalar-tensor theories, but extends also to
debated or unknown scenarios, such as f(R) and Lanczos-Lovelock theories. We
thus provide new tools going beyond the standard methods, where the latter turn
out to be inconclusive or inapplicable.
