  A wealth of observations recently challenged the notion of a universal
stellar initial mass function (IMF) by showing evidences in favour of a
variability of this statistical indicator as a function of galaxy properties. I
present predictions from the semi-analytic model GAEA (GAlaxy Evolution and
Assembly), which features (a) a detailed treatment of chemical enrichment, (b)
an improved stellar feedback scheme, and (c) implements theoretical
prescriptions for IMF variations. Our variable IMF realizations predict
intrinsic stellar masses and mass-to-light ratios larger than those estimated
from synthetic photometry assuming a universal IMF. This provides a
self-consistent interpretation for the observed mismatch between
photometrically inferred stellar masses of local early-type galaxies and those
derived by dynamical and spectroscopic studies. At higher redshifts, the
assumption of a variable IMF has a deep impact on our ability to reconstruct
the evolution of the galaxy stellar mass function and the star formation
history of galaxies.
