Calculations using double ζ plus polarization basis sets or effective core potentials and two configuration MCSCF wave functions augmented by multireference singles and doubles configuration interaction are used to investigate the effect of various Substituents on the singlet triplet splitting in methylene. Of particular interest is the conclusion that when CH2SiH3 or CH2GeH3 is a substituent, the cumulative effect of the polarizability of the C-Si and C-Ge bonds and the rotation of these bonds into the appropriate alignment required for effective electron donation into the carbene π orbital results in preferential stabilization of the corresponding singlet carbenes, a β-silicon (or germanium) effect analogous to that which is well-known in cation chemistry.