The variation of the mass-to-light M/L ratios of early-type galaxies as a function of their luminosities L is investigated. It is shown that the tilt β=0.27 (in the B band) of the fundamental plane relation M/L~Lβ can be understood as a combination of two effects: about one-quarter (i.e., Δβ=0.07) is a result of systematic variations of the stellar population properties with increasing luminosity. The remaining three-quarters (i.e., Δβ=0.2) can be completely attributed to nonhomology effects that lead to a systematic change of the surface brightness profiles with increasing luminosity. Consequently, the observed tilt in the K band (β=0.17), where stellar population effects are negligible, is explained by nonhomology effects alone. After correcting for nonhomology, the mean value of the mass-to-light ratio of elliptical galaxies (M/LB) is 7.1+/-2.8 (1 σ scatter).