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The stellar initial mass function (IMF) is a fundamental property in themeasurement of stellar masses and galaxy star formation histories. In this workwe focus on the most massive galaxies in the nearby universe$\log(M_{\star}/M_{\odot})>11.2$. We obtain high quality Magellan/LDSS-3 longslit spectroscopy with a wide wavelength coverage of $0.4\mu{\rm m}-1.01\mu{\rmm}$ for 41 early-type galaxies (ETGs) in the MASSIVE survey, and derive highS/N spectra within an aperture of $R_{\rm e}/8$. Using detailed stellarsynthesis models, we constrain the elemental abundances and stellar IMF of eachgalaxy through full spectral modeling. All the ETGs in our sample have an IMFthat is steeper than a Milky Way (Kroupa) IMF. The best-fit IMF mismatchparameter, $\alpha_{\rm IMF}=(M/L)/(M/L)_{\rm MW}$, ranges from 1.12 to 3.05,with an average of $\langle \alpha_{\rm IMF} \rangle=1.84$, suggesting that onaverage, the IMF is more bottom-heavy than Salpeter. Comparing the estimatedstellar mass with the dynamical mass, we find that most galaxies have stellarmasses smaller than their dynamical masses within the $1\sigma$ uncertainty. Wecomplement our sample with lower-mass galaxies from the literature, and confirmthat $\log(\alpha_{\rm IMF})$ is positively correlated with $\log(\sigma)$,$\log(M_{\star})$, and $\log(M_{\rm dyn})$. The IMF in the centers of moremassive ETGs is more bottom-heavy. In addition, we find that $\log(\alpha_{\rmIMF})$ is positively correlated with both [Mg/Fe] and the estimated totalmetallicity [Z/H]. We find suggestive evidence that the effective stellarsurface density $\Sigma_{\rm Kroupa}$ might be responsible for the variation of$\alpha_{\rm IMF}$. We conclude that $\sigma$, [Mg/Fe] and [Z/H] are theprimary drivers of the global stellar IMF variation.

The MASSIVE SURVEY XVI. The Stellar Initial Mass Function in the Center of MASSIVE Early-Type Galaxies