  From a study of the integrated light properties of 200 globular clusters
(GCs) in M31, Strader et al. found that the mass-to-light ratios are lower than
what is expected from simple stellar population (SSP) models with a `canonical'
stellar initial mass function (IMF), with the discrepancy being larger at high
metallicities. We use dynamical multi-mass models, that include a prescription
for equipartition, to quantify the bias in the inferred dynamical mass as the
result of the assumption that light follows mass. For a universal IMF and a
metallicity dependent present day mass function we find that the inferred mass
from integrated light properties systematically under estimates the true mass,
and that the bias is more important at high metallicities, as was found for the
M31 GCs. We show that mass segregation and a flattening of the mass function
have opposing effects of similar magnitude on the mass inferred from integrated
properties. This makes the mass-to-light ratio as derived from integrated
properties an inadequate probe of the low-mass end of the stellar mass
function. There is, therefore, no need for variations in the IMF, nor the need
to invoke depletion of low-mass stars, to explain the observations. Finally, we
find that the retention fraction of stellar-mass black holes (BHs) is an
equally important parameter in understanding the mass segregation bias. We
speculatively put forward to idea that kinematical data of GCs can in fact be
used to constrain the total mass in stellar-mass BHs in GCs.
