The electronic structure of the lowest excited triplet states of C-60 and C-70 are characterized by a magnetic interaction between the unpaired electrons for which the zero-field-splitting parameter D is negative for the former and positive for the latter molecule. The sign of D has so far been qualitatively understood, and its magnitude has been found to critically depend on the degree of delocalization of the singly occupied molecular orbitals. In this contribution, the effect of spin polarization to the zero-field-splitting parameters of the fullerenes is evaluated, the inclusion of which results in quantitative agreement between the experimental and calculated D values. The direct spin- spin contribution is found to be dominant for both molecules. For C-60 a significant contribution of 20% of the total zero-field splitting has been found to derive from spin polarization. The physical reason for the sign difference of D for C-60 and C-70 is traced back to the relative phases of the local p(z) orbitals or adjacent carbon atoms near the equatorial plane in both singly occupied molecular orbitals. These relative phases differ for C-60 and C-70, because C-70 has an additional set of ten carbon atoms in its equatorial plane as compared to C-60. Additionally, the triplet wave function of C-70 is found to contain significant multireference character. In order to evaluate the effect of spin polarization in rnultireference systems. the zero-field-splitting parameters of the lowest triplet state of benzene have been evaluated in an illustrative and insightful calculation its well. Though this prototypical molecule is much smaller than C-60 and C-70, the electronic structure of its lowest excited triplet state is also of multireference character. For benzene, 18% of the total zero-field splitting arises from spin polarization.