The perovskites LaMnO3 and CaFeO3 consisting of high-spin d(4) transition metal ions undergo different types of distortions, i.e., a Jahn-Teller distortion in LaMnO3 and a charge disproportionation in CaFeO3, We investigated the electronic factor causing this difference on the basis of first principles spin-polarized electronic band structure calculations for their ideal cubic structures and also tight-binding electronic band structure calculations for their ideal cubic and distorted structures. Our study shows that a charge disproportionation is favored over a Jahn-Teller distortion in CaFeO3 because the covalent character is strong in the Fe-O bond, while the opposite is true for LaMnO3 because the covalent character is weak in the Mn-O bond. In spin-polarized electronic band structure calculations, the covalency of the M-O (M = Fe, Mn) bond is enhanced in the up-spin bands but is reduced in the down-spin bands. Our analysis shows that electron-electron repulsion causes the energy gap between the metal 3d and the oxygen 2p orbitals to become larger for the down-spin than for the up-spin-orbital interactions. Thus in the d-block e(g) bands of both LaMnO3 and CaFeO3 the metal 3d orbital contribution is larger in the down-spin than in the up-spin bands.