The initial mass function (IMF) of early-type galaxies (ETGs) has been found to feature systematic variations by both dynamical and spectroscopic studies. In particular, spectral line strengths, based on gravity-sensitive features, suggest an excess of low-mass stars in massive ETGs, i.e. a bottom-heavy IMF. The physical drivers of IMF variations are currently unknown. The abundance ratio of α elements, such as [Mg/Fe], has been suggested as a possible driver of the IMF changes, although dynamical constraints do not support this claim. In this letter, we take advantage of the large Sloan Digital Sky Survey data base. Our sample comprises 24 781 high-quality spectra, covering a large range in velocity dispersion (100 <σ0 < 320 km s-1) and abundance ratio ( - 0.1 < [Mg/Fe] < +0.4). The large volume of data allows us to stack the spectra at fixed values of σ0 and [Mg/Fe]. Our analysis - based on gravity-sensitive line strengths - gives a strong correlation with central velocity dispersion and a negligible variation with [Mg/Fe] at fixed σ0. This result is robust against individual elemental abundance variations, and seems not to raise any apparent inconsistency with the alternative method based on galaxy dynamics.