Additive models for molecular parameters have a long history, and remain important for the rapid prediction of molecular properties. To test the validity of an additive approach, we study the polarizabilities of methyl branched alkanes in the terahertz spectral range, and compare these to the polarizabilities of their straight chain counterparts. A single branched methyl group increases the isotropic polarizability by a fixed amount, regardless of the carbon backbone chain length or the methyl branch's position. These results, which are also compared to DFT calculations, establish the validity of an empirical additive approach for these prototype non-polar and non-hydrogen-bonding liquids. (C) 2013 Elsevier B.V. All rights reserved.