Using a magnetohydrodynamics numerical code that includes the description of density and pressure evolution and anisotropic thermal conductivity, it is shown that single-helicity states in the reversed-field pinch (RFP) configuration are obtained when the resistivity has a radial profile sharply increasing close to the wall. In contrast, a uniform resistivity produces multiple-helicity states. A radially increasing resistivity profile is determined in RFP experiments by the temperature difference between the plasma core and the wall. The results of the simulations presented in this letter are an indication that quasi-single-helicity states observed in experiments with high toroidal currents may be a consequence of the resistivity profile determined by the high temperatures reached in the plasma core in high-current discharges.