  The ferroelectric order and magnetic field induced effects observed in the
spiral phase of MnWO4 are described theoretically. It is demonstrated
explicitly that the Dzyaloshinskii-Moriya antisymmetric interactions contribute
to the correlation between spins and electric dipoles in the incommensurate and
commensurate ferroelectric phases of magnetic multiferroics. However, other
single-site symmetric interactions are shown to be involved in the
magnetoelectric process, suggesting the possible existence of an electric
polarization originating from purely symmetric effects.
