  We present a simple calculation leading to the quantum
gravitationally-induced decoherence of a spatial superposition of a massive
object in the linear coupling regime. The point of this calculation is to
illustrate that the gravitationally-induced collapse could be of the same
origin as any other collapse, i.e. due to the entanglement between the system
(here a massive object) and its environment (in this case gravity, but it could
well be the electromagnetic or any other field). We then point out that, in
some cases, one has to be careful when concluding that matter-wave interference
of large masses (to be quantified) would be prevented by their coupling to the
gravitational field. We discuss how to experimentally discriminate between
decoherence due to entanglement, decoherence due to classical dephasig as well
as a genuine collapse of quantum superpositions (if such a process exists at
all).
