Second-harmonic generation in chiral molecules has been thoroughly studied, especially in surface experiments. On a molecular viewpoint, the linear optical properties of chiral molecules necessitate to include the magnetic dipole response, but such is not the case for second-order nonlinear optics where fully electric-dipole response is sufficient. We propose here a full quantum-mechanical calculation of the hyperpolarizability of chiral molecules in the "one-electron" model which shows that (i) such chiral molecules are inherently nonlinear and (i) for nonlinear response, achiral and chiral contributions can have the same order of magnitude, contrarily to what is observed in linear optics.