  Within a galaxy the stellar mass-to-light ratio $\Upsilon_*$ is not constant.
Spatially resolved kinematics of nearby early-type galaxies suggest that
allowing for a variable initial mass function (IMF) returns significantly
larger $\Upsilon_*$ gradients than if the IMF is held fixed. If $\Upsilon_*$ is
greater in the central regions, then ignoring the IMF-driven gradient can
overestimate $M_*^{\rm dyn}$ by as much as a factor of two for the most massive
galaxies, though stellar population estimates $M_*^{\rm SP}$ are also affected.
Large $\Upsilon_*$-gradients have four main consequences: First, $M_*^{\rm
dyn}$ cannot be estimated independently of stellar population synthesis models.
Second, if there is a lower limit to $\Upsilon_*$ and gradients are unknown,
then requiring $M_*^{\rm dyn}=M_*^{\rm SP}$ constrains them. Third, if
gradients are stronger in more massive galaxies, then $M_*^{\rm dyn}$ and
$M_*^{\rm SP}$ can be brought into agreement, not by shifting $M_*^{\rm SP}$
upwards by invoking constant bottom-heavy IMFs, as advocated by a number of
recent studies, but by revising $M_*^{\rm dyn}$ estimates in the literature
downwards. Fourth, accounting for $\Upsilon_*$ gradients changes the high-mass
slope of the stellar mass function $\phi(M_*^{\rm dyn})$, and reduces the
associated stellar mass density. These conclusions potentially impact estimates
of the need for feedback and adiabatic contraction, so our results highlight
the importance of measuring $\Upsilon_*$ gradients in larger samples.
